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_virtual_functions.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 import pytest
 
 import env  # noqa: F401
@@ -10,12 +9,12 @@ from pybind11_tests import ConstructorStats  # noqa: E402
 def test_override(capture, msg):
     class ExtendedExampleVirt(m.ExampleVirt):
         def __init__(self, state):
-            super(ExtendedExampleVirt, self).__init__(state + 1)
+            super().__init__(state + 1)
             self.data = "Hello world"
 
         def run(self, value):
             print("ExtendedExampleVirt::run(%i), calling parent.." % value)
-            return super(ExtendedExampleVirt, self).run(value + 1)
+            return super().run(value + 1)
 
         def run_bool(self):
             print("ExtendedExampleVirt::run_bool()")
@@ -29,7 +28,7 @@ def test_override(capture, msg):
 
     class ExtendedExampleVirt2(ExtendedExampleVirt):
         def __init__(self, state):
-            super(ExtendedExampleVirt2, self).__init__(state + 1)
+            super().__init__(state + 1)
 
         def get_string2(self):
             return "override2"
@@ -41,7 +40,7 @@ def test_override(capture, msg):
         capture
         == """
         Original implementation of ExampleVirt::run(state=10, value=20, str1=default1, str2=default2)
-    """  # noqa: E501 line too long
+    """
     )
 
     with pytest.raises(RuntimeError) as excinfo:
@@ -59,7 +58,7 @@ def test_override(capture, msg):
         == """
         ExtendedExampleVirt::run(20), calling parent..
         Original implementation of ExampleVirt::run(state=11, value=21, str1=override1, str2=default2)
-    """  # noqa: E501 line too long
+    """
     )
     with capture:
         assert m.runExampleVirtBool(ex12p) is False
@@ -76,7 +75,7 @@ def test_override(capture, msg):
         == """
         ExtendedExampleVirt::run(50), calling parent..
         Original implementation of ExampleVirt::run(state=17, value=51, str1=override1, str2=override2)
-    """  # noqa: E501 line too long
+    """
     )
 
     cstats = ConstructorStats.get(m.ExampleVirt)
@@ -97,7 +96,7 @@ def test_alias_delay_initialization1(capture):
 
     class B(m.A):
         def __init__(self):
-            super(B, self).__init__()
+            super().__init__()
 
         def f(self):
             print("In python f()")
@@ -137,7 +136,7 @@ def test_alias_delay_initialization2(capture):
 
     class B2(m.A2):
         def __init__(self):
-            super(B2, self).__init__()
+            super().__init__()
 
         def f(self):
             print("In python B2.f()")
@@ -245,7 +244,7 @@ def test_dispatch_issue(msg):
     class PyClass2(m.DispatchIssue):
         def dispatch(self):
             with pytest.raises(RuntimeError) as excinfo:
-                super(PyClass2, self).dispatch()
+                super().dispatch()
             assert (
                 msg(excinfo.value)
                 == 'Tried to call pure virtual function "Base::dispatch"'
@@ -262,7 +261,7 @@ def test_recursive_dispatch_issue(msg):
 
     class Data(m.Data):
         def __init__(self, value):
-            super(Data, self).__init__()
+            super().__init__()
             self.value = value
 
     class Adder(m.Adder):