Add typing, remove deprecated RelatedLinkColumn

[jieter]

Writing union types as X | Y is first added in python 3.10, if we keep supporting it until the end of life of python 3.9, that would mean we can only merge this after 2025-10.

Alternatively, we can use the Union[X, Y] syntax which is already supported by python 3.9.

Another issue is Unpack which is available from python 3.11. It is part of typing_extensions, so we can still use it.

The final few errors in the current state of this PR is this list (in which the tests are not yet included in the type checking):

$ mypy django_tables2/
django_tables2/columns/base.py:327: error: Dict entry 0 has incompatible type "str": "tuple[str, Union[list[Any], tuple[Any, ...]]]"; expected "str": "Callable[..., str]"  [dict-item]
django_tables2/columns/base.py:329: error: Dict entry 0 has incompatible type "str": "Optional[Accessor]"; expected "str": "Callable[..., str]"  [dict-item]
django_tables2/columns/base.py:332: error: Argument 2 to "LinkTransform" has incompatible type "**dict[str, Callable[..., str]]"; expected "Optional[Accessor]"  [arg-type]
django_tables2/columns/base.py:332: error: Argument 2 to "LinkTransform" has incompatible type "**dict[str, Callable[..., str]]"; expected "Union[list[Any], tuple[Any, ...], None]"  [arg-type]
django_tables2/columns/base.py:388: error: Incompatible return value type (got "Union[SafeString, QuerySet[Any, Any]]", expected "SafeString")  [return-value]
django_tables2/columns/base.py:404: error: Argument 2 to "call_with_appropriate" has incompatible type "CellArguments"; expected "dict[str, Any]"  [arg-type]
django_tables2/columns/base.py:706: error: Incompatible types in assignment (expression has type "Union[Any, str, _StrPromise]", variable has type "str")  [assignment]
django_tables2/columns/manytomanycolumn.py:94: error: Argument 1 to "filter" of "ManyToManyColumn" has incompatible type "Union[SafeString, QuerySet[Any, Any]]"; expected "QuerySet[Any, Any]"  [arg-type]
django_tables2/rows.py:155: error: Incompatible types in assignment (expression has type "None", variable has type "str")  [assignment]
django_tables2/rows.py:169: error: Item "None" of "Optional[Any]" has no attribute "exists"  [union-attr]
django_tables2/rows.py:183: error: "BoundRow" has no attribute "column"  [attr-defined]
django_tables2/rows.py:184: error: Incompatible types (expression has type "BoundRow", TypedDict item "bound_column" has type "BoundColumn")  [typeddict-item]
django_tables2/views.py:172: error: Definition of "get_paginate_by" in base class "TableMixinBase" is incompatible with definition in base class "MultipleObjectMixin"  [misc]
django_tables2/views.py:245: error: "MultiTableMixin" has no attribute "request"  [attr-defined]
django_tables2/export/views.py:52: error: "ExportMixin" has no attribute "get_table"  [attr-defined]
django_tables2/export/views.py:52: error: "ExportMixin" has no attribute "get_table_kwargs"; maybe "get_dataset_kwargs"?  [attr-defined]
Found 16 errors in 5 files (checked 27 source **files)**

Ideally, users of django-tables2 should still be able to use def render(self, record) or def render(self, value) without complaints of the type checker. I think this might be hard to achieve, so we should document that using def render(self, **kwargs: Unpack[CellArguments]) and unpacking kwargs in the body of the method is the way to achieve type safety.

[mschoettle]

With pyupgrade it should be easy to move from Union to | once Python 3.9 support gets dropped. It looks like the pre-commit config needs a small update to target 3.9 instead of 3.8.