Drop attrs dependency, use dataclasses instead (#10669)

Since pytest now requires Python>=3.7, we can use the stdlib attrs
clone, dataclasses, instead of the OG package.

attrs is still somewhat nicer than dataclasses and has some extra
functionality, but for pytest usage there's not really a justification
IMO to impose the extra dependency on users when a standard alternative
exists.

testing/acceptance_test.py

@@ -1,9 +1,8 @@
+import dataclasses
 import os
 import sys
 import types
 
-import attr
-
 import pytest
 from _pytest.compat import importlib_metadata
 from _pytest.config import ExitCode
@@ -115,11 +114,11 @@ class TestGeneralUsage:
 
         loaded = []
 
-        @attr.s
+        @dataclasses.dataclass
         class DummyEntryPoint:
-            name = attr.ib()
-            module = attr.ib()
-            group = "pytest11"
+            name: str
+            module: str
+            group: str = "pytest11"
 
             def load(self):
                 __import__(self.module)
@@ -132,10 +131,10 @@ class TestGeneralUsage:
             DummyEntryPoint("mycov", "mycov_module"),
         ]
 
-        @attr.s
+        @dataclasses.dataclass
         class DummyDist:
-            entry_points = attr.ib()
-            files = ()
+            entry_points: object
+            files: object = ()
 
         def my_dists():
             return (DummyDist(entry_points),)
@@ -1037,14 +1036,14 @@ def test_fixture_values_leak(pytester: Pytester) -> None:
     """
     pytester.makepyfile(
         """
-        import attr
+        import dataclasses
         import gc
         import pytest
         import weakref
 
-        @attr.s
-        class SomeObj(object):
-            name = attr.ib()
+        @dataclasses.dataclass
+        class SomeObj:
+            name: str
 
         fix_of_test1_ref = None
         session_ref = None