The `--with-system-ffi` configure option is redundant on Linux for Python 3.7+

[edmorley]

Hi!

The Dockerfiles for Linux in this repository currently pass --with-system-ffi to configure:

python/Dockerfile-linux.template

Line 162 in 1999778

--with-system-ffi \

This option was added back in #205 / #358.

However, as of Python 3.7+ using the system ffi library is the default for Linux:
https://bugs.python.org/issue27979
python/cpython@f40d4dd#diff-49473dca262eeab3b4a43002adb08b4db31020d190caaad1594b47f1d5daa810

As such, builds are currently emitting this warning during configure:

checking for --with-system-ffi... yes
configure: WARNING: --with(out)-system-ffi is ignored on this platform

Examples in CI on this repo:

• Python 3.7.13: https://github.com/docker-library/python/runs/5876761054?check_suite_focus=true#step:5:926
• Python 3.11.0a7: https://github.com/docker-library/python/runs/5876759514?check_suite_focus=true#step:5:999

Given that this repo no longer supports Python versions older than Python 3.7, it seems this option can now be dropped?