Add default values for third-party stubs beginning with 'P' (#9957)

stubs/pyRFC3339/pyrfc3339/generator.pyi

@@ -1,3 +1,3 @@
 from datetime import datetime
 
-def generate(dt: datetime, utc: bool = ..., accept_naive: bool = ..., microseconds: bool = ...) -> str: ...
+def generate(dt: datetime, utc: bool = True, accept_naive: bool = False, microseconds: bool = False) -> str: ...

stubs/pyRFC3339/pyrfc3339/parser.pyi

@@ -1,3 +1,3 @@
 from datetime import datetime
 
-def parse(timestamp: str, utc: bool = ..., produce_naive: bool = ...) -> datetime: ...
+def parse(timestamp: str, utc: bool = False, produce_naive: bool = False) -> datetime: ...