Python: 3.6 - 3.9

[ax3l]

Add support for Python 3.9, drop support for 3.5 (end-of-life since 9/2020).

Notes:

• we already ship Python 3.9 via conda-forge for the latest release
• theoretically, CPython 3.9.0 has a bug on macOS and does not work with pybind11. pybind11 2.6.0 ships a work-around and CPython 3.9.0 on conda-forge as well as release CPython 3.9.1 will ship a fix: https://bugs.python.org/issue42015 bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure m_self is kept alive long enough python/cpython#22670
• we already test Python 3.9 on CI (macOS builds from homebrew, with CPython 3.9.0_2)