chore: Add Version.namespace, remove import_namespace

[dangotbanned]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Related feat: Adds private Namespace class #2324

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

Waaaaaaaay simpler and avoids needing to have an unreachable branch for the Enum.

The much bigger change I wanna make is replacing import_dtypes_module -> Version.dtypes.
Thought I'd float this one first, since:

• that PR would touch 21 modules
• require configuring/removing

  narwhals/.pre-commit-config.yaml

  Lines 62 to 81 in f47ad1f

  	- id: dtypes-import
  	name: don't import from narwhals.dtypes (use `import_dtypes_module` instead)
  	entry: |
  	(?x)
  	import\ narwhals.dtypes|
  	from\ narwhals\ import\ dtypes|
  	from\ narwhals.dtypes\ import\ [^D_]+|
  	import\ narwhals.stable.v1.dtypes|
  	from\ narwhals.stable.v1\ import\ dtypes|
  	from\ narwhals.stable.v1.dtypes\ import
  	language: pygrep
  	files: ^narwhals/
  	exclude: |
  	(?x)
  	^(
  	narwhals/utils\.py|
  	narwhals/stable/v1/_dtypes.py|
  	narwhals/.*__init__.py|
  	narwhals/.*typing\.py
  	)

[FBruzzesi]

Nice, thanks @dangotbanned 👌🏼