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/test_iostream.py

@@ -1,44 +1,8 @@
-# -*- coding: utf-8 -*-
-import sys
-from contextlib import contextmanager
+from contextlib import redirect_stderr, redirect_stdout
+from io import StringIO
 
 from pybind11_tests import iostream as m
 
-try:
-    # Python 3
-    from io import StringIO
-except ImportError:
-    # Python 2
-    try:
-        from cStringIO import StringIO
-    except ImportError:
-        from StringIO import StringIO
-
-try:
-    # Python 3.4
-    from contextlib import redirect_stdout
-except ImportError:
-
-    @contextmanager
-    def redirect_stdout(target):
-        original = sys.stdout
-        sys.stdout = target
-        yield
-        sys.stdout = original
-
-
-try:
-    # Python 3.5
-    from contextlib import redirect_stderr
-except ImportError:
-
-    @contextmanager
-    def redirect_stderr(target):
-        original = sys.stderr
-        sys.stderr = target
-        yield
-        sys.stderr = original
-
 
 def test_captured(capsys):
     msg = "I've been redirected to Python, I hope!"