feat: remove Python 3.6 support (#5177)

* Change Python version guard: PYTHON < 3.7 IS UNSUPPORTED.

* Replace or remove Python 3.6 jobs.

* Move appveyor to Python 3.8

* Change `[tool.pylint]` `master.py-version` from `3.6` to `3.8`

* Change `[tool.pylint]` `master.py-version` to `3.7`

* Remove `centos:7` job; Change almalinux:8 job to use Python 3.8

* Try 🐍 3.8 • ubuntu-20.04 • x64 without `-DCMAKE_CXX_FLAGS="-D_=1"`

* Update setup.cfg as suggested by @henryiii

* Try running `cmake --build . --target cpptest` on all platforms (`standard` job).

* Disable deadsnakes jobs entirely.

* Apply PR #5179: Add Python 3.10, 3.11, 3.12 to win32 job matrix.

* Add back `-DCMAKE_CXX_FLAGS="-D_=1"` but do not install boost in that case.

* PY_VERSION_HEX < 3.7 cleanup pass: include/pybind11

* WITH_THREAD cleanup pass: include/pybind11

* Undo incorrect change.

* Revert "Disable deadsnakes jobs entirely."

This reverts commit bbcd0087b2.

* WITH_THREAD cleanup pass: tests/

* Change Python version guard in pybind11/__init__.py: pybind11 does not support Python < 3.7.

* Misc cleanup pass

* chore: use future imports

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Update tests/test_numpy_array.py

* Update test_numpy_array.py

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

setup.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 # Setup script for PyPI; use CMakeFile.txt to build extension modules
+from __future__ import annotations
 
 import contextlib
 import os
@@ -9,9 +10,9 @@ import shutil
 import string
 import subprocess
 import sys
+from collections.abc import Generator
 from pathlib import Path
 from tempfile import TemporaryDirectory
-from typing import Dict, Iterator, List, Union
 
 import setuptools.command.sdist
 
@@ -23,7 +24,7 @@ VERSION_FILE = Path("pybind11/_version.py")
 COMMON_FILE = Path("include/pybind11/detail/common.h")
 
 
-def build_expected_version_hex(matches: Dict[str, str]) -> str:
+def build_expected_version_hex(matches: dict[str, str]) -> str:
     patch_level_serial = matches["PATCH"]
     serial = None
     major = int(matches["MAJOR"])
@@ -64,7 +65,7 @@ to_src = (
 
 
 # Read the listed version
-loc: Dict[str, str] = {}
+loc: dict[str, str] = {}
 code = compile(VERSION_FILE.read_text(encoding="utf-8"), "pybind11/_version.py", "exec")
 exec(code, loc)
 version = loc["__version__"]
@@ -84,9 +85,7 @@ if version_hex != exp_version_hex:
 
 
 # TODO: use literals & overload (typing extensions or Python 3.8)
-def get_and_replace(
-    filename: Path, binary: bool = False, **opts: str
-) -> Union[bytes, str]:
+def get_and_replace(filename: Path, binary: bool = False, **opts: str) -> bytes | str:
     if binary:
         contents = filename.read_bytes()
         return string.Template(contents.decode()).substitute(opts).encode()
@@ -97,7 +96,7 @@ def get_and_replace(
 # Use our input files instead when making the SDist (and anything that depends
 # on it, like a wheel)
 class SDist(setuptools.command.sdist.sdist):
-    def make_release_tree(self, base_dir: str, files: List[str]) -> None:
+    def make_release_tree(self, base_dir: str, files: list[str]) -> None:
         super().make_release_tree(base_dir, files)
 
         for to, src in to_src:
@@ -112,7 +111,7 @@ class SDist(setuptools.command.sdist.sdist):
 
 # Remove the CMake install directory when done
 @contextlib.contextmanager
-def remove_output(*sources: str) -> Iterator[None]:
+def remove_output(*sources: str) -> Generator[None, None, None]:
     try:
         yield
     finally: