Fix #180 (#181)

Only use typing extensions if truly needed.
We should eventually add a CI test without all the de dependencies, because they probably bring in typing extensions in other versions....

plum/signature.py

@@ -1,10 +1,16 @@
 import inspect
 import operator
+import sys
 from copy import copy
 from typing import Any, Callable, ClassVar, List, Set, Tuple, Union
 
+# TODO: When minimum version required is 3.11, remove typing extensions
+if sys.version_info >= (3, 11):  # pragma: specific no cover 3.7 3.8 3.9 3.10
+    from typing import Self
+else:  # pragma: specific no cover 3.11
+    from typing_extensions import Self
+
 from rich.segment import Segment
-from typing_extensions import Self
 
 import beartype.door
 from beartype.peps import resolve_pep563 as beartype_resolve_pep563