Permit arbitrary whitespace around versions specifier in parenthesis

This is more consistent with the rest of the format which is largely
whitespace agnostic.

packaging/_parser.py

@@ -197,11 +197,13 @@ def _parse_extras_list(tokenizer: Tokenizer) -> List[str]:
 
 def _parse_specifier(tokenizer: Tokenizer) -> str:
     """
-    specifier = LEFT_PARENTHESIS version_many RIGHT_PARENTHESIS
-              | version_many
+    specifier = LEFT_PARENTHESIS WS? version_many WS? RIGHT_PARENTHESIS
+              | WS? version_many WS?
     """
     with tokenizer.enclosing_tokens("LEFT_PARENTHESIS", "RIGHT_PARENTHESIS"):
+        tokenizer.consume("WS")
         parsed_specifiers = _parse_version_many(tokenizer)
+        tokenizer.consume("WS")
 
     return parsed_specifiers
 

tests/test_requirements.py

@@ -79,9 +79,9 @@
         ("git+https://git.example.com/[email protected]", ""),
         ("git+https://git.example.com/MyProject.git@refs/pull/123/head", ""),
         (None, "==={ws}arbitrarystring"),
-        (None, "(==={ws}arbitrarystring)"),
+        (None, "({ws}==={ws}arbitrarystring{ws})"),
         (None, "=={ws}1.0"),
-        (None, "(=={ws}1.0)"),
+        (None, "({ws}=={ws}1.0{ws})"),
         (None, "<={ws}1!3.0.0.rc2"),
         (None, ">{ws}2.2{ws},{ws}<{ws}3"),
         (None, "(>{ws}2.2{ws},{ws}<{ws}3)"),