Officially drop support for Python 3.9

[srittau]

Part of #13782

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

kornia (https://github.com/kornia/kornia)
+ kornia/contrib/diamond_square.py:213: error: Incompatible types in assignment (expression has type "float", variable has type "int")  [assignment]
+ kornia/contrib/diamond_square.py:214: error: Incompatible types in assignment (expression has type "float", variable has type "int")  [assignment]

rotki (https://github.com/rotki/rotki)
+ rotkehlchen/inquirer.py:1300: error: Incompatible return value type (got "FVal", expected "Price | None")  [return-value]

sympy (https://github.com/sympy/sympy)
+ sympy/core/evalf.py:1356: error: Incompatible types in assignment (expression has type "int", variable has type "Symbol")  [assignment]
+ sympy/polys/rings.py:3458: error: Incompatible types in assignment (expression has type "float", variable has type "int")  [assignment]
+ sympy/polys/rings.py:3460: error: Incompatible types in assignment (expression has type "PolyElement[Er]", variable has type "int")  [assignment]
+ sympy/polys/rings.py:3462: error: No overload variant of "exquo" of "PolyElement" matches argument type "int"  [call-overload]
+ sympy/polys/rings.py:3462: note: Possible overload variants:
+ sympy/polys/rings.py:3462: note:     def exquo(self, G: list[PolyElement[Er]]) -> list[PolyElement[Er]]
+ sympy/polys/rings.py:3462: note:     def exquo(self, G: PolyElement[Er]) -> PolyElement[Er]
+ sympy/polys/rings.py:3464: error: Argument 2 to "coeff_wrt" of "PolyElement" has incompatible type "float"; expected "int"  [arg-type]

imagehash (https://github.com/JohannesBuchner/imagehash)
+ imagehash/__init__.py:444: error: Incompatible types in assignment (expression has type "int", variable has type "ndarray[tuple[int], dtype[Any]]")  [assignment]

core (https://github.com/home-assistant/core)
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
+ homeassistant/components/isy994/helpers.py:452: error: Redundant cast to "float"  [redundant-cast]

nox (https://github.com/wntrblm/nox)
+ nox/_option_set.py:50: error: Only concrete class can be given where "type[PathLike[Any]]" is expected  [type-abstract]

zulip (https://github.com/zulip/zulip)
- ...typeshed_to_test/stdlib/builtins.pyi:118: note: "SubTest" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:110: note: "SubTest" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:118: note: "FileFixture" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:110: note: "FileFixture" defined here

ibis (https://github.com/ibis-project/ibis)
- ...typeshed_to_test/stdlib/builtins.pyi:118: note: "Any" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:110: note: "Any" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:118: note: "__init__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:110: note: "__init__" of "object" defined here

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/reshape/concat.py:861: error: Unused "type: ignore" comment  [unused-ignore]

prefect (https://github.com/PrefectHQ/prefect)
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here

strawberry (https://github.com/strawberry-graphql/strawberry)
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here
- ...typeshed_to_test/stdlib/builtins.pyi:140: note: "__init_subclass__" of "object" defined here
+ ...typeshed_to_test/stdlib/builtins.pyi:132: note: "__init_subclass__" of "object" defined here

discord.py (https://github.com/Rapptz/discord.py)
- ...typeshed_to_test/stdlib/typing.pyi:1049: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1025: note: "update" of "TypedDict" defined here