[3.8] bpo-38270: Check for hash digest algorithms and avoid MD5 (GH-16382) (GH-16393)

[3.8] bpo-38270: Check for hash digest algorithms and avoid MD5 (GH-16382) (GH-16393)

Make it easier to run and test Python on systems with restrict crypto policies:

* add requires_hashdigest to test.support to check if a hash digest algorithm is available and working
* avoid MD5 in test_hmac
* replace MD5 with SHA256 in test_tarfile
* mark network tests that require MD5 for MD5-based digest auth or CRAM-MD5

https://bugs.python.org/issue38270
(cherry picked from commit c64a1a61e6fc542cada40eb069a239317e1af36e)

Co-authored-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue38270

Automerge-Triggered-By: @tiran