It has been a year since m2crypto merged python3 support. See pull request: https://gitlab.com/m2crypto/m2crypto/merge_requests/65 Gentoo's package is still python-2.7 only
This should be dealt with by upgrading m2crypto to the latest upstream version, which is 0.30.1 . An update to this newer version will allow this bug to be closed off, as this support was committed in 0.28.0 . I have been testing 0.30.1 for the last day with python-3.6 and it seems to be OK.
lastest version of m2crypto is pushed into ::ixit overlay meanwhile it gets pulled into gentoo x86, if you have any tested patch for python3 support feel free to send patch or send pull request. Thanks
m2crypto-0.31.0-r1.ebuild is in the tree and resolves this issue. This new version supports python3_* in both the package and the ebuild, so I think we can safely close/resolve this bug report.