Install optional dependencies during test stage

[sethmlarson]

The test phase of the build pipeline is missing:

• libmpdec
• tkinter

It would be nice to copy what's being done on the CPython repository so we can properly run the entire test suite.

[JelleZijlstra]

For what it's worth I looked through https://github.com/python/release-tools/actions/runs/8992970829/job/24703944115 and looked at all skipped tests. Most are Windows-specific and obviously can't run on the Linux runner, but I noticed the following that sound like they could possibly work (in addition to decimal and tkinter):

test_ioctl skipped -- Unable to open /dev/tty

• Maybe depends on actual TTY?

test_generated_cases skipped -- cases_generator directory could not be found
test_clinic skipped -- clinic directory could not be found

• Maybe these only work if running in the whole repo?

test_dbm_gnu skipped -- No module named '_gdbm'
test_dbm_ndbm skipped -- No module named '_dbm'

• Judging from the devguide, something like apt-get install libgdbm-dev should work

[erlend-aasland]

See also .github/workflows/posix-deps-apt.sh in the cpython repo.