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>

tests/extra_setuptools/test_setuphelper.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 import os
 import subprocess
 import sys
@@ -19,7 +18,7 @@ def test_simple_setup_py(monkeypatch, tmpdir, parallel, std):
 
     (tmpdir / "setup.py").write_text(
         dedent(
-            u"""\
+            """\
             import sys
             sys.path.append({MAIN_DIR!r})
 
@@ -58,7 +57,7 @@ def test_simple_setup_py(monkeypatch, tmpdir, parallel, std):
 
     (tmpdir / "main.cpp").write_text(
         dedent(
-            u"""\
+            """\
             #include <pybind11/pybind11.h>
 
             int f(int x) {
@@ -96,7 +95,7 @@ def test_simple_setup_py(monkeypatch, tmpdir, parallel, std):
 
     (tmpdir / "test.py").write_text(
         dedent(
-            u"""\
+            """\
             import simple_setup
             assert simple_setup.f(3) == 9
             """
@@ -121,10 +120,11 @@ def test_intree_extensions(monkeypatch, tmpdir):
     subdir.ensure_dir()
     src = subdir / "ext.cpp"
     src.ensure()
-    (ext,) = intree_extensions([src.relto(tmpdir)])
+    relpath = src.relto(tmpdir)
+    (ext,) = intree_extensions([relpath])
     assert ext.name == "ext"
     subdir.ensure("__init__.py")
-    (ext,) = intree_extensions([src.relto(tmpdir)])
+    (ext,) = intree_extensions([relpath])
     assert ext.name == "dir.ext"