Add SoftDeprecationWarning warning category #106137

Implementation of my proposed PEP 387 "Soft Deprecation" policy: python/peps#3182

• Add SoftDeprecationWarning warning category
• Ignore these warnings by default, except in the __main__ module (similar to PEP 565: Show DeprecationWarning in main)

As an example, I propose to soft deprecate the getopt module: PR #105735 (currently, the PR uses a regular "hard" deprecation).

Linked PRs

• gh-106137: Add SoftDeprecationWarning category #106142