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>

docs/advanced/classes.rst

@@ -133,14 +133,14 @@ a virtual method call.
     >>> from example import *
     >>> d = Dog()
     >>> call_go(d)
-    u'woof! woof! woof! '
+    'woof! woof! woof! '
     >>> class Cat(Animal):
     ...     def go(self, n_times):
     ...         return "meow! " * n_times
     ...
     >>> c = Cat()
     >>> call_go(c)
-    u'meow! meow! meow! '
+    'meow! meow! meow! '
 
 If you are defining a custom constructor in a derived Python class, you *must*
 ensure that you explicitly call the bound C++ constructor using ``__init__``,
@@ -813,26 +813,21 @@ An instance can now be pickled as follows:
 
 .. code-block:: python
 
-    try:
-        import cPickle as pickle  # Use cPickle on Python 2.7
-    except ImportError:
-        import pickle
+    import pickle
 
     p = Pickleable("test_value")
     p.setExtra(15)
-    data = pickle.dumps(p, 2)
+    data = pickle.dumps(p)
 
 
 .. note::
-    Note that only the cPickle module is supported on Python 2.7.
-
-    The second argument to ``dumps`` is also crucial: it selects the pickle
-    protocol version 2, since the older version 1 is not supported. Newer
-    versions are also fine—for instance, specify ``-1`` to always use the
-    latest available version. Beware: failure to follow these instructions
-    will cause important pybind11 memory allocation routines to be skipped
-    during unpickling, which will likely lead to memory corruption and/or
-    segmentation faults.
+    If given, the second argument to ``dumps`` must be 2 or larger - 0 and 1 are
+    not supported. Newer versions are also fine; for instance, specify ``-1`` to
+    always use the latest available version. Beware: failure to follow these
+    instructions will cause important pybind11 memory allocation routines to be
+    skipped during unpickling, which will likely lead to memory corruption
+    and/or segmentation faults. Python defaults to version 3 (Python 3-3.7) and
+    version 4 for Python 3.8+.
 
 .. seealso::
 
@@ -849,11 +844,9 @@ Python normally uses references in assignments. Sometimes a real copy is needed
 to prevent changing all copies. The ``copy`` module [#f5]_ provides these
 capabilities.
 
-On Python 3, a class with pickle support is automatically also (deep)copy
+A class with pickle support is automatically also (deep)copy
 compatible. However, performance can be improved by adding custom
-``__copy__`` and ``__deepcopy__`` methods. With Python 2.7, these custom methods
-are mandatory for (deep)copy compatibility, because pybind11 only supports
-cPickle.
+``__copy__`` and ``__deepcopy__`` methods.
 
 For simple classes (deep)copy can be enabled by using the copy constructor,
 which should look as follows: