Testing zfec with pypy3 on Windows has been failing, as reported in #38. Disabling pypy3 testing on Windows alone does not appear to be straightforward with GitHub Actions. This PR drops pypy3 from the test matrix, until a time when things get better.
This PR drops Python 3.5 from the test matrix, because py35 has been EOL-ed.
Adds newer Python3 versions and pypy to tox envlist, and drops Python 3.5 from it.
Changes: