gh-145028: Fix blake2 tests in test_hashlib when it is missing due to configure --without-builtin-hashlib-hashes

[tucif]

This PR addresses Issue #145028 tracking test_hashlib failure on cpython configured --without-builtin-hashlib-hashes

Changes in test_hashlib.py

• Guard testing of blake2 with existing utils to verify its existence (@require_blake2)
• Remove blake2 variations from algorithms in constructor when _blake2 is None, as they may already be in that list through its initialization from self.supported_hashnames.
• Add subtests in test_algorithms_available and skip if ValueError is raised, so it's easy to tell what exactly was skipped and why.
• Add skip_if_blake2_not_builtin helper

Testing with fix on python without builtin blake2

python -m unittest Lib.test.test_hashlib.HashLibTestCase
...
----------------------------------------------------------------------
Ran 82 tests in 7.005s

OK (skipped=29)

Sample skips on test_algorithms_available

  test_algorithms_available (Lib.test.test_hashlib.HashLibTestCase.test_algorithms_available) [blake2b] ... skipped 'unsupported hash algorithm blake2b'
  test_algorithms_available (Lib.test.test_hashlib.HashLibTestCase.test_algorithms_available) [blake2s] ... skipped 'unsupported hash algorithm blake2s'

• Issue: test_hashlib blake2 tests fail on python configured --without-builtin-hashlib-hashes #145028

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[miss-islington-app]

Thanks @tucif for the PR, and @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @tucif and @gpshead, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 273d5062ca17ac47354486f3fc6e672a04cf22e0 3.14