Compile Python 3.9 instead of using package Python 3.6

[radarhere]

python-pillow/Pillow#5768 suggests removing Ubuntu 18.04 and all CentOS versions as part of removing Python 3.6.

This PR suggests compiling Python 3.9 from source on those platforms instead.

[hugovk]

The images are quite a bit bigger:

Before

REPOSITORY                      TAG         IMAGE ID       CREATED              SIZE
***/centos-7-amd64              main        05b2c4974886   About a minute ago   697MB
***/centos-8-amd64              main        e6f162c3b1e5   2 minutes ago        669MB
***/centos-stream-8-amd64       main        1a2a9533ad39   2 minutes ago        905MB
***/ubuntu-18.04-bionic-amd64   main        7139e14f07ac   2 minutes ago        898MB

After

REPOSITORY                               TAG    IMAGE ID       CREATED              SIZE
githubactions/centos-7-amd64             main   d0a7ed7490c7   About a minute ago   1.12GB
githubactions/centos-8-amd64             main   06a8403ce0ed   2 minutes ago        1.21GB
githubactions/centos-stream-8-amd64      main   fdb87ea65f93   2 minutes ago        1.45GB
githubactions/ubuntu-18.04-bionic-amd64  main   df2be4cec2d9   3 minutes ago        1.32GB

Are there any temp files we can delete? At least Python-3.9.7.tgz can be deleted first, but maybe some C compilation stuff?

There's some official Python Dockerfiles linked here (not CentOS/Ubuntu):

• https://hub.docker.com/_/python

For example:

• https://github.com/docker-library/python/blob/a7ff0b769c0b79b619ee93a9308f9a013bd66e7a/3.9/buster/slim/Dockerfile

[hugovk]

Now

REPOSITORY                               TAG    IMAGE ID       CREATED              SIZE
githubactions/centos-7-amd64             main   35d2b19599bb   About a minute ago   860MB
githubactions/centos-8-amd64             main   fe46254cae96   2 minutes ago        918MB
githubactions/centos-stream-8-amd64      main   a0fc63941944   About a minute ago   1.15GB
githubactions/ubuntu-18.04-bionic-amd64  main   bed3a39facef   2 minutes ago        1.05GB

👍