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.

src/_pytest/legacypath.py

@@ -1,4 +1,5 @@
 """Add backward compatibility support for the legacy py path type."""
+import dataclasses
 import shlex
 import subprocess
 from pathlib import Path
@@ -7,7 +8,6 @@ from typing import Optional
 from typing import TYPE_CHECKING
 from typing import Union
 
-import attr
 from iniconfig import SectionWrapper
 
 from _pytest.cacheprovider import Cache
@@ -268,7 +268,7 @@ class LegacyTestdirPlugin:
 
 
 @final
-@attr.s(init=False, auto_attribs=True)
+@dataclasses.dataclass
 class TempdirFactory:
     """Backward compatibility wrapper that implements :class:`py.path.local`
     for :class:`TempPathFactory`.