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/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: |