Not sure whether this is intended behavior (since the only ebuild in 3.7 slot is ~keyworded).
Here is the emerge --info output: Portage 2.3.40 (python 3.6.6-final-0, default/linux/amd64/17.0/desktop, gcc-7.3.0, glibc-2.26-r7, 4.17.2-gentoo x86_64) ================================================================= System uname: Linux-4.17.2-gentoo-x86_64-Intel-R-_Core-TM-_i5-4460_CPU_@_3.20GHz-with-gentoo-2.4.1 KiB Mem: 8078352 total, 1887884 free KiB Swap: 0 total, 0 free Timestamp of repository gentoo: Sat, 07 Jul 2018 17:00:01 +0000 Head commit of repository gentoo: 6cf4e0ed8928c01552e84a22a439e0edf5e0093f sh bash 4.4_p12 ld GNU ld (Gentoo 2.30 p2) 2.30.0 app-shells/bash: 4.4_p12::gentoo dev-java/java-config: 2.2.0-r4::gentoo dev-lang/perl: 5.24.3-r1::gentoo dev-lang/python: 2.7.15::gentoo, 3.4.8-r1::gentoo, 3.5.5-r1::gentoo, 3.6.6::gentoo, 3.7.0::gentoo dev-util/cmake: 3.9.6::gentoo dev-util/pkgconfig: 0.29.2::gentoo sys-apps/baselayout: 2.4.1-r2::gentoo sys-apps/openrc: 0.34.11::gentoo sys-apps/sandbox: 2.13::gentoo sys-devel/autoconf: 2.13::gentoo, 2.69-r4::gentoo sys-devel/automake: 1.15.1-r2::gentoo sys-devel/binutils: 2.29.1-r1::gentoo, 2.30-r2::gentoo sys-devel/gcc: 6.4.0-r1::gentoo, 7.3.0-r3::gentoo sys-devel/gcc-config: 1.8-r1::gentoo sys-devel/libtool: 2.4.6-r3::gentoo sys-devel/make: 4.2.1::gentoo sys-kernel/linux-headers: 4.13::gentoo (virtual/os-headers) sys-libs/glibc: 2.26-r7::gentoo Repositories: gentoo location: /usr/portage sync-type: rsync sync-uri: rsync://rsync.gentoo.org/gentoo-portage priority: -1000 sync-rsync-verify-jobs: 1 sync-rsync-verify-metamanifest: yes sync-rsync-verify-max-age: 24 sync-rsync-extra-opts: eroen location: /var/lib/layman/eroen masters: gentoo priority: 0 dotnet location: /var/lib/layman/dotnet masters: gentoo priority: 1 eclipse location: /var/lib/layman/eclipse masters: gentoo priority: 2 ACCEPT_KEYWORDS="amd64" ACCEPT_LICENSE="* -@EULA" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-O2 -pipe" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/share/gnupg/qualified.txt" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/php/apache2-php7.2/ext-active/ /etc/php/cgi-php7.2/ext-active/ /etc/php/cli-php7.2/ext-active/ /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo /etc/texmf/language.dat.d /etc/texmf/language.def.d /etc/texmf/updmap.d /etc/texmf/web2c" CXXFLAGS="-O2 -pipe" DISTDIR="/usr/portage/distfiles" ENV_UNSET="DBUS_SESSION_BUS_ADDRESS DISPLAY PERL5LIB PERL5OPT PERLPREFIX PERL_CORE PERL_MB_OPT PERL_MM_OPT XAUTHORITY XDG_CACHE_HOME XDG_CONFIG_HOME XDG_DATA_HOME XDG_RUNTIME_DIR" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync multilib-strict news parallel-fetch preserve-libs protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync xattr" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="http://distfiles.gentoo.org" LANG="en_US.utf8" LDFLAGS="-Wl,-O1 -Wl,--as-needed" LINGUAS="pl" PKGDIR="/usr/portage/packages" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages --exclude=/.git" PORTAGE_TMPDIR="/var/tmp" USE="X a52 aac acl acpi alsa amd64 berkdb bluetooth branding bzip2 cairo cdda cdr cli consolekit crypt cups cxx dbus dri dts dvd dvdr emboss encode exif fam flac fortran gdbm gif glamor gpm gtk iconv ipv6 jpeg lcms ldap libnotify libtirpc mad mng modules mp3 mp4 mpeg multilib ncurses nls nptl offensive ogg opengl openmp pam pango pcre pdf png policykit ppds qt5 readline sdl seccomp spell ssl startup-notification svg tcpd tiff tk truetype udev udisks unicode upower usb vorbis wxwidgets x264 xattr xcb xml xv xvid zlib" ABI_X86="64" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" APACHE2_MODULES="authn_core authz_core socache_shmcb unixd actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias" CALLIGRA_FEATURES="karbon plan sheets stage words" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="mmx mmxext sse sse2 sse3 ssse3 sse4_1 sse4_2" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock isync itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf skytraq superstar2 timing tsip tripmate tnt ublox ubx" GRUB_PLATFORMS="efi-64" INPUT_DEVICES="evdev" KERNEL="linux" L10N="pl" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php7-2" POSTGRES_TARGETS="postgres9_5 postgres10" PYTHON_SINGLE_TARGET="python3_6" PYTHON_TARGETS="python3_7 python3_6 python3_5 python3_4 python2_7" RUBY_TARGETS="ruby25 ruby23" USERLAND="GNU" VIDEO_CARDS="radeon" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account" Unset: CC, CPPFLAGS, CTARGET, CXX, EMERGE_DEFAULT_OPTS, INSTALL_MASK, LC_ALL, MAKEOPTS, PORTAGE_BINHOST, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPT
you need to rebuild dev-lang/python-exec to use eselect for python 3.7
