Add Self in xarray.core.types

[Illviljan]

Adds typing_extensions.Self wrapped in some safety checks.

Wont really become useful until python/mypy#11871 is fixed. But it can be used with pyright at least.

[Illviljan]

The https://peps.python.org/pep-0673/ has a few clear use cases. Here's a simple example with this PR uncommented:

if TYPE_CHECKING:
    from xarray.core.types import Self


class Foo:
    def returns_self(self) -> Self:
        return 1

!pyright G:\Program\Dropbox\Python\mypy_self_type.py
No configuration file found.stubPath G:\Program\Dropbox\Python\typings is not a valid directory.

No pyproject.toml file found.
Assuming Python platform Windows
Searching for source files
Found 1 source file
pyright 1.1.266
G:\Program\Dropbox\Python\mypy_self_type.py
  G:\Program\Dropbox\Python\mypy_self_type.py:45:35 - error: "Self" is not exported from module "xarray.core.types"
  Â Â Import from "typing_extensions" instead (reportPrivateImportUsage)
  G:\Program\Dropbox\Python\mypy_self_type.py:50:16 - error: Expression of type "Literal[1]" cannot be assigned to return type "Self@Foo"
  Â Â Type "int" cannot be assigned to type "Foo"
  Â Â Â Â "int" is incompatible with "Foo" (reportGeneralTypeIssues)
2 errors, 0 warnings, 0 informations 
Completed in 0.778sec

Wasn't that many obvious examples where this could be used now that I started to look for it... But here's a few ones where it could be used.

xarray/xarray/core/indexes.py

Lines 102 to 103 in 60f8c3d

	) -> Index:
	return self

xarray/xarray/backends/common.py

Lines 133 to 134 in 6b1d97a

	def __enter__(self):
	return self

xarray/xarray/plot/facetgrid.py

Line 297 in fbaf815

return self