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_kwargs_and_defaults.py

@@ -1,7 +1,5 @@
-# -*- coding: utf-8 -*-
 import pytest
 
-import env  # noqa: F401
 from pybind11_tests import kwargs_and_defaults as m
 
 
@@ -82,7 +80,7 @@ def test_mixed_args_and_kwargs(msg):
             1. (arg0: int, arg1: float, *args) -> tuple
 
         Invoked with: 1
-    """  # noqa: E501 line too long
+    """
     )
     with pytest.raises(TypeError) as excinfo:
         assert mpa()
@@ -93,7 +91,7 @@ def test_mixed_args_and_kwargs(msg):
             1. (arg0: int, arg1: float, *args) -> tuple
 
         Invoked with:
-    """  # noqa: E501 line too long
+    """
     )
 
     assert mpk(-2, 3.5, pi=3.14159, e=2.71828) == (
@@ -127,7 +125,7 @@ def test_mixed_args_and_kwargs(msg):
             1. (i: int = 1, j: float = 3.14159, *args, **kwargs) -> tuple
 
         Invoked with: 1; kwargs: i=1
-    """  # noqa: E501 line too long
+    """
     )
     with pytest.raises(TypeError) as excinfo:
         assert mpakd(1, 2, j=1)
@@ -138,7 +136,7 @@ def test_mixed_args_and_kwargs(msg):
             1. (i: int = 1, j: float = 3.14159, *args, **kwargs) -> tuple
 
         Invoked with: 1, 2; kwargs: j=1
-    """  # noqa: E501 line too long
+    """
     )
 
     # Arguments after a py::args are automatically keyword-only (pybind 2.9+)
@@ -343,7 +341,6 @@ def test_signatures():
     )
 
 
-@pytest.mark.xfail("env.PYPY and env.PY2", reason="PyPy2 doesn't double count")
 def test_args_refcount():
     """Issue/PR #1216 - py::args elements get double-inc_ref()ed when combined with regular
     arguments"""