Stop supporting Python 3.6

[reneleonhardt]

Python 3.7 final has been released, support for 3.6 can be dropped.

[reneleonhardt]

Can I remove all #if PY_VERSION_HEX < 0x03070000 blocks and #if PY_VERSION_HEX >= 0x03070000 #else blocks?

[reneleonhardt]

I see there are manylinux2014 wheels built against GLIBC_2.17.
Would it be possible to build manylinux_2_24 and manylinux_2_28 too?

[mrabarnett]

I'll probably drop support for Python 3.6 when I add support for Python 3.12, which is currently in beta.

If you want other Linux wheels, you'll need to tell me exactly what needs to be done in main.yml.

[reneleonhardt]

I was just wondering if newer manylinux builds would be possible and welcome after I saw the comments regarding maximum compatibility and memcpy, but pypi showed GLIBC 2.17 builds.
pypa/manylinux states support for manylinux_2_24 (EOL)... but also that support ended in January 🙈
manylinux_2_28 is the currently supported platform.

Could we try and test moving the python versions according to the pip compatibility list in an new PR?

Python	manylinux	fallback	Python release dates
3.6	manylinux1		22 Dec 2016
3.7 3.8	manylinux2014	manylinux2010	26 Jun 2018, 14 Oct 2019
3.9	manylinux_2_24	manylinux2014	05 Oct 2020
3.10 3.11 3.12	manylinux_2_28	manylinux_2_24	04 Oct 2021, 24 Oct 2022, prerelease

[reneleonhardt]

I created #500 as an example for manylinux_2_28 (please don't read the branch name 🙈).