python: skip pytest_pycollect_makeitem work on certain names

When a Python object (module/class/instance) is collected, for each name
in `obj.__dict__` (and up its MRO) the pytest_pycollect_makeitem hook is
called for potentially creating a node for it.

These Python objects have a bunch of builtin attributes that are
extremely unlikely to be collected. But due to their pervasiveness,
dispatching the hook for them ends up being mildly expensive and also
pollutes PYTEST_DEBUG=1 output and such.

Let's just ignore these attributes.

On the pandas test suite commit 04e9e0afd476b1b8bed930e47bf60e,
collect only, irrelevant lines snipped, about 5% improvement:

Before:

```
         51195095 function calls (48844352 primitive calls) in 39.089 seconds

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
226602/54    0.145    0.000   38.940    0.721 manager.py:90(_hookexec)
    72227    0.285    0.000   20.146    0.000 python.py:424(_makeitem)
    72227    0.171    0.000   16.678    0.000 python.py:218(pytest_pycollect_makeitem)
```

After:

```
          48410921 function calls (46240870 primitive calls) in 36.950 seconds

    ncalls  tottime  percall  cumtime  percall filename:lineno(function)
 181429/54    0.113    0.000   36.777    0.681 manager.py:90(_hookexec)
     27054    0.130    0.000   17.755    0.001 python.py:465(_makeitem)
     27054    0.121    0.000   16.219    0.001 python.py:218(pytest_pycollect_makeitem)
```

changelog/7671.trivial.rst

@@ -0,0 +1,6 @@
+When collecting tests, pytest finds test classes and functions by examining the
+attributes of python objects (modules, classes and instances). To speed up this
+process, pytest now ignores builtin attributes (like ``__class__``,
+``__delattr__`` and ``__new__``) without consulting the ``python_classes`` and
+``python_functions`` configuration options and without passing them to plugins
+using the ``pytest_pycollect_makeitem`` hook.

src/_pytest/python.py

@@ -5,6 +5,7 @@
 import itertools
 import os
 import sys
+import types
 import typing
 import warnings
 from collections import Counter
@@ -343,6 +344,26 @@ def reportinfo(self) -> Tuple[Union[py.path.local, str], int, str]:
         return fspath, lineno, modpath
 
 
+# As an optimization, these builtin attribute names are pre-ignored when
+# iterating over an object during collection -- the pytest_pycollect_makeitem
+# hook is not called for them.
+# fmt: off
+class _EmptyClass: pass  # noqa: E701
+IGNORED_ATTRIBUTES = frozenset.union(  # noqa: E305
+    frozenset(),
+    # Module.
+    dir(types.ModuleType("empty_module")),
+    # Some extra module attributes the above doesn't catch.
+    {"__builtins__", "__file__", "__cached__"},
+    # Class.
+    dir(_EmptyClass),
+    # Instance.
+    dir(_EmptyClass()),
+)
+del _EmptyClass
+# fmt: on
+
+
 class PyCollector(PyobjMixin, nodes.Collector):
     def funcnamefilter(self, name: str) -> bool:
         return self._matches_prefix_or_glob_option("python_functions", name)
@@ -404,6 +425,8 @@ def collect(self) -> Iterable[Union[nodes.Item, nodes.Collector]]:
             # Note: seems like the dict can change during iteration -
             # be careful not to remove the list() without consideration.
             for name, obj in list(dic.items()):
+                if name in IGNORED_ATTRIBUTES:
+                    continue
                 if name in seen:
                     continue
                 seen.add(name)

testing/python/collect.py

@@ -885,6 +885,34 @@ def test_something():
         result = testdir.runpytest_subprocess()
         result.stdout.fnmatch_lines(["*1 passed*"])
 
+    def test_early_ignored_attributes(self, testdir: Testdir) -> None:
+        """Builtin attributes should be ignored early on, even if
+        configuration would otherwise allow them.
+
+        This tests a performance optimization, not correctness, really,
+        although it tests PytestCollectionWarning is not raised, while
+        it would have been raised otherwise.
+        """
+        testdir.makeini(
+            """
+            [pytest]
+            python_classes=*
+            python_functions=*
+        """
+        )
+        testdir.makepyfile(
+            """
+            class TestEmpty:
+                pass
+            test_empty = TestEmpty()
+            def test_real():
+                pass
+        """
+        )
+        items, rec = testdir.inline_genitems()
+        assert rec.ret == 0
+        assert len(items) == 1
+
 
 def test_setup_only_available_in_subdir(testdir):
     sub1 = testdir.mkpydir("sub1")