Python 2 removal part 1: tests (C++ code is intentionally ~untouched) (#3688)

* `#error BYE_BYE_GOLDEN_SNAKE`

* Removing everything related to 2.7 from ci.yml

* Commenting-out Centos7

* Removing `PYTHON: 27` from .appveyor.yml

* "PY2" removal, mainly from tests. C++ code is not touched.

* Systematic removal of `u` prefix from `u"..."` and `u'...'` literals. Collateral cleanup of a couple minor other things.

* Cleaning up around case-insensitive hits for `[^a-z]py.*2` in tests/.

* Removing obsolete Python 2 mention in compiling.rst

* Proper `#error` for Python 2.

* Using PY_VERSION_HEX to guard `#error "PYTHON 2 IS NO LONGER SUPPORTED.`

* chore: bump pre-commit

* style: run pre-commit for pyupgrade 3+

* tests: use sys.version_info, not PY

* chore: more Python 2 removal

* Uncommenting Centos7 block (PR #3691 showed that it is working again).

* Update pre-commit hooks

* Fix pre-commit hook

* refactor: remove Python 2 from CMake

* refactor: remove Python 2 from setup code

* refactor: simplify, better static typing

* feat: fail with nice messages

* refactor: drop Python 2 C++ code

* docs: cleanup for Python 3

* revert: intree

revert: intree

* docs: minor touchup to py2 statement

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

.github/workflows/ci.yml

@@ -25,7 +25,6 @@ jobs:
       matrix:
         runs-on: [ubuntu-latest, windows-2022, macos-latest]
         python:
-        - '2.7'
         - '3.5'
         - '3.6'
         - '3.9'
@@ -49,13 +48,9 @@ jobs:
             python: '3.6'
             args: >
               -DPYBIND11_FINDPYTHON=ON
-          - runs-on: macos-latest
-            python: 'pypy-2.7'
           # Inject a couple Windows 2019 runs
           - runs-on: windows-2019
             python: '3.9'
-          - runs-on: windows-2019
-            python: '2.7'
 
     name: "🐍 ${{ matrix.python }} • ${{ matrix.runs-on }} • x64 ${{ matrix.args }}"
     runs-on: ${{ matrix.runs-on }}
@@ -168,22 +163,6 @@ jobs:
     - name: Interface test
       run: cmake --build build2 --target test_cmake_build
 
-    # Eventually Microsoft might have an action for setting up
-    # MSVC, but for now, this action works:
-    - name: Prepare compiler environment for Windows 🐍 2.7
-      if: matrix.python == 2.7 && runner.os == 'Windows'
-      uses: ilammy/msvc-dev-cmd@v1.10.0
-      with:
-        arch: x64
-
-    # This makes two environment variables available in the following step(s)
-    - name: Set Windows 🐍 2.7 environment variables
-      if: matrix.python == 2.7 && runner.os == 'Windows'
-      shell: bash
-      run: |
-        echo "DISTUTILS_USE_SDK=1" >> $GITHUB_ENV
-        echo "MSSdk=1" >> $GITHUB_ENV
-
     # This makes sure the setup_helpers module can build packages using
     # setuptools
     - name: Setuptools helpers test
@@ -784,7 +763,6 @@ jobs:
       fail-fast: false
       matrix:
         python:
-          - 2.7
           - 3.6
           - 3.7
           # todo: check/cpptest does not support 3.8+ yet
@@ -832,17 +810,12 @@ jobs:
       fail-fast: false
       matrix:
         python:
-          - 2.7
           - 3.5
           - 3.7
         std:
           - 14
 
         include:
-          - python: 2.7
-            std: 17
-            args: >
-              -DCMAKE_CXX_FLAGS="/permissive- /EHsc /GR"
           - python: 3.7
             std: 17
             args: >

.github/workflows/pip.yml

@@ -17,19 +17,19 @@ env:
 
 jobs:
   # This builds the sdists and wheels and makes sure the files are exactly as
-  # expected. Using Windows and Python 2.7, since that is often the most
+  # expected. Using Windows and Python 3.6, since that is often the most
   # challenging matrix element.
   test-packaging:
-    name: 🐍 2.7 • 📦 tests • windows-latest
+    name: 🐍 3.6 • 📦 tests • windows-latest
     runs-on: windows-latest
 
     steps:
     - uses: actions/checkout@v2
 
-    - name: Setup 🐍 2.7
+    - name: Setup 🐍 3.6
       uses: actions/setup-python@v2
       with:
-        python-version: 2.7
+        python-version: 3.6
 
     - name: Prepare env
       run: |