Fix typing for specifiers.BaseSpecifier.filter() (#643)

Signed-off-by: Henry Schreiner <[email protected]>

src/packaging/specifiers.py

@@ -11,12 +11,23 @@
 import abc
 import itertools
 import re
-from typing import Callable, Iterable, Iterator, List, Optional, Set, Tuple, Union
+from typing import (
+    Callable,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    TypeVar,
+    Union,
+)
 
 from .utils import canonicalize_version
 from .version import Version
 
 UnparsedVersion = Union[Version, str]
+UnparsedVersionVar = TypeVar("UnparsedVersionVar", bound=UnparsedVersion)
 CallableOperator = Callable[[Version, str], bool]
 
 
@@ -85,8 +96,8 @@ def contains(self, item: str, prereleases: Optional[bool] = None) -> bool:
 
     @abc.abstractmethod
     def filter(
-        self, iterable: Iterable[UnparsedVersion], prereleases: Optional[bool] = None
-    ) -> Iterator[UnparsedVersion]:
+        self, iterable: Iterable[UnparsedVersionVar], prereleases: Optional[bool] = None
+    ) -> Iterator[UnparsedVersionVar]:
         """
         Takes an iterable of items and filters them so that only items which
         are contained within this specifier are allowed in it.
@@ -565,8 +576,8 @@ def contains(
         return operator_callable(normalized_item, self.version)
 
     def filter(
-        self, iterable: Iterable[UnparsedVersion], prereleases: Optional[bool] = None
-    ) -> Iterator[UnparsedVersion]:
+        self, iterable: Iterable[UnparsedVersionVar], prereleases: Optional[bool] = None
+    ) -> Iterator[UnparsedVersionVar]:
         """Filter items in the given iterable, that match the specifier.
 
         :param iterable:
@@ -915,8 +926,8 @@ def contains(
         return all(s.contains(item, prereleases=prereleases) for s in self._specs)
 
     def filter(
-        self, iterable: Iterable[UnparsedVersion], prereleases: Optional[bool] = None
-    ) -> Iterator[UnparsedVersion]:
+        self, iterable: Iterable[UnparsedVersionVar], prereleases: Optional[bool] = None
+    ) -> Iterator[UnparsedVersionVar]:
         """Filter items in the given iterable, that match the specifiers in this set.
 
         :param iterable:
@@ -972,8 +983,8 @@ def filter(
         # which will filter out any pre-releases, unless there are no final
         # releases.
         else:
-            filtered: List[UnparsedVersion] = []
-            found_prereleases: List[UnparsedVersion] = []
+            filtered: List[UnparsedVersionVar] = []
+            found_prereleases: List[UnparsedVersionVar] = []
 
             for item in iterable:
                 parsed_version = _coerce_version(item)