According to the kdevelop python configuration dialog, the pycodestyle package is required for pep8 checking within kdevelop.
Sorry, that's dev-util/kdevelop-python-5.1.0 dependent on dev-python/pycodestyle (specific version unknown to me)
Thanks for your report, fixed in kdevelop-python-5.1.0-r1 (git commit 413d79af2fc39379aac57e657e29f645e3fd09fe)