Sync with importlib_metadata for Python 3.16 #160386

Tracking issue for applying importlib_metadata changes to importlib.metadata in 3.16, continuing the pattern of #132947 (3.15) and #117089 (3.13).

This covers importlib_metadata 10.0.0, a change in functionality rather than a fix:

• Shared protocols for consumers. IDistribution and IPackagePath join
  the existing PackageMetadata in _meta.py, and the functions returning
  distributions and files are annotated with them. A consumer can target one
  interface across importlib.metadata and the backport instead of either
  library's concrete classes, which is what a custom provider's objects
  require: the provider serves consumers of both libraries and cannot subclass
  its way into satisfying both (API incompatibility with importlib.metadata (or at least the API is not type-safe?) importlib_metadata#486,
  Enforce local versions of objects and introduce shared protocols importlib_metadata#505). All three protocols are
  @runtime_checkable, so isinstance answers the question without
  evaluating a distribution's properties on 3.12 and later.

  What does not come downstream is the other half of that release. The backport
  substitutes its own classes for the importlib.metadata objects a provider
  supplies, and warns where a provider's class omits a member IDistribution
  requires; both live in its _compat package, which is excluded from the
  port. Only the protocols those mechanisms are defined against are synced, so
  the guidance for providers is documented here as the backport's behavior
  (Expand documentation for importlib.metadata custom providers to include caveats regarding backports #123976).

Linked PRs

• gh-160386: Apply changes from importlib_metadata 9.1 #160387
• gh-160386: Apply changes from importlib_metadata 10.0 and document the shared protocols #160344