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>

.github/workflows/pip.yml

@@ -21,19 +21,18 @@ env:
 
 jobs:
   # This builds the sdists and wheels and makes sure the files are exactly as
-  # expected. Using Windows and Python 3.6, since that is often the most
-  # challenging matrix element.
+  # expected.
   test-packaging:
-    name: 🐍 3.6 • 📦 tests • windows-latest
+    name: 🐍 3.8 • 📦 tests • windows-latest
     runs-on: windows-latest
 
     steps:
     - uses: actions/checkout@v4
 
-    - name: Setup 🐍 3.6
+    - name: Setup 🐍 3.8
       uses: actions/setup-python@v5
       with:
-        python-version: 3.6
+        python-version: 3.8
 
     - name: Prepare env
       run: |