manager: deprecate register(None)

[MbappeWU]

Fixes #790.

Problem

Since #752, PluginManager.register(None) is silently ignored (it used to misbehave, #731, and raising broke devpi's tests, #749). In #749 the follow-up agreed was to add a deprecation warning so callers can migrate before None becomes an error.

Change

• register(None) still returns None and registers nothing, but now emits a DeprecationWarning with stacklevel=2, so the warning points at the caller's register() line:

  Passing None to PluginManager.register() is deprecated and will become an error in a future version; it is currently ignored.

• The register docstring mentions the deprecation.

• test_register_ignores_none asserts the warning (message, a single record, and that it is attributed to the calling file) in addition to the existing "ignored" assertions.

• Towncrier fragment changelog/790.removal.rst ("Deprecations and Removals").

I used a plain DeprecationWarning, as the hookspec-without-self deprecation does. Happy to switch to a PluggyWarning subclass if you prefer a pluggy-specific category.

Verification

• uv run pytest: 219 passed.
• With the _manager.py change reverted, the updated test fails: Failed: DID NOT WARN. No warnings of type (<class 'DeprecationWarning'>,) were emitted.
• uv run pre-commit run on the changed files: ruff check/format, autoflake, pyupgrade, blacken-docs and mypy all pass.