ENH: Change python version to 3.6.12

[BishopWolf]

Current python version 3.6.7 has not received any update since October 20,2018. All bug fixes for branch 3.6 are integrated in version 3.6.12 which is still receiving corrections (the last one from August 17, 2020)

Some bugs from version 3.6.7 are rising and exposed here

[jcfr]

Thanks for the contribution 🙏

Would be great if you could:

• fix the commit message
• submit a PR to support this new version. See https://github.com/python-cmake-buildsystem/python-cmake-buildsystem/blob/a1ce7c2cf346ce62648d8c3e9e5cae7f655d4309/CMakeLists.txt#L201-L209

[BishopWolf]

What is the requirement for the commit message? I have tried like 5 different approaches and neither pass the check

[jcfr]

What is the requirement for the commit message?

See https://slicer.readthedocs.io/en/latest/developer_guide/contributing.html#how-to-write-commit-messages

[BishopWolf]

I have added support for python 3.6.8 to 3.6.12 in the PR

[BishopWolf]

Tested in Docker and Ubuntu. It runs smoothly!

[jcfr]

Thanks for testing on Ubuntu and submitting a PR on the upstream project.

After updating the CI, it turns out that windows build is failing:
https://open.cdash.org/index.php?project=CPython&date=2020-09-03&filtercount=1&showfilters=1&field1=buildname&compare1=63&value1=3.6.12-VS

error C2065: 'PY3_DLLNAME': undeclared identifier

It may be worth looking if some changes from python-cmake-buildsystem/python-cmake-buildsystem#267 are relevant.

[BishopWolf]

@jcfr Unfortunately I am very bad at windows debugging. May somebody else help on this?

This PR just moves from python 3.6.7 to python 3.6.12. In theory there shall not be any change in functionality inside the same branch

I have added the missing line in the PR

[BishopWolf]

It seems that line solved the build errors! ;)

[lassoan]

I'm closing this issue now, as Python-3.6 will reach end of life in about a month. We are trying to upgrade to Python-3.9 (#6026).