chore(typing): add CompliantSeries.dtype

[dangotbanned]

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

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

Related issues

• Related chore(typing): add CompliantExpr._version #2060
• fix TypeVar used in (SparkLike|DuckDB)Expr  #2044 (comment)

Checklist

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

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

Will simplify a workaround I've done in #2064 (comment)

Code block

narwhals/narwhals/_selectors.py

Lines 45 to 51 in 3c5ea87

	# NOTE: Plugging the gap of this not being defined in `CompliantSeries`
	class CompliantSeriesWithDType(CompliantSeries, Protocol):
	@property
	def dtype(self) -> DType: ...
	SeriesT = TypeVar("SeriesT", bound="CompliantSeriesWithDType")

The property is available on all CompliantSeries already, but not visible to a type checker without this

[MarcoGorelli]