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/test_tmpdir.py

@@ -1,3 +1,4 @@
+import dataclasses
 import os
 import stat
 import sys
@@ -6,8 +7,7 @@ from pathlib import Path
 from typing import Callable
 from typing import cast
 from typing import List
-
-import attr
+from typing import Union
 
 import pytest
 from _pytest import pathlib
@@ -31,9 +31,9 @@ def test_tmp_path_fixture(pytester: Pytester) -> None:
     results.stdout.fnmatch_lines(["*1 passed*"])
 
 
-@attr.s
+@dataclasses.dataclass
 class FakeConfig:
-    basetemp = attr.ib()
+    basetemp: Union[str, Path]
 
     @property
     def trace(self):
@@ -56,7 +56,7 @@ class FakeConfig:
 
 
 class TestTmpPathHandler:
-    def test_mktemp(self, tmp_path):
+    def test_mktemp(self, tmp_path: Path) -> None:
         config = cast(Config, FakeConfig(tmp_path))
         t = TempPathFactory.from_config(config, _ispytest=True)
         tmp = t.mktemp("world")
@@ -67,7 +67,9 @@ class TestTmpPathHandler:
         assert str(tmp2.relative_to(t.getbasetemp())).startswith("this")
         assert tmp2 != tmp
 
-    def test_tmppath_relative_basetemp_absolute(self, tmp_path, monkeypatch):
+    def test_tmppath_relative_basetemp_absolute(
+        self, tmp_path: Path, monkeypatch: MonkeyPatch
+    ) -> None:
         """#4425"""
         monkeypatch.chdir(tmp_path)
         config = cast(Config, FakeConfig("hello"))