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/conftest.py

@@ -1,3 +1,4 @@
+import dataclasses
 import re
 import sys
 from typing import List
@@ -192,20 +193,18 @@ def mock_timing(monkeypatch: MonkeyPatch):
     Time is static, and only advances through `sleep` calls, thus tests might sleep over large
     numbers and obtain accurate time() calls at the end, making tests reliable and instant.
     """
-    import attr
 
-    @attr.s
+    @dataclasses.dataclass
     class MockTiming:
+        _current_time: float = 1590150050.0
 
-        _current_time = attr.ib(default=1590150050.0)
-
-        def sleep(self, seconds):
+        def sleep(self, seconds: float) -> None:
             self._current_time += seconds
 
-        def time(self):
+        def time(self) -> float:
             return self._current_time
 
-        def patch(self):
+        def patch(self) -> None:
             from _pytest import timing
 
             monkeypatch.setattr(timing, "sleep", self.sleep)