Fix datetime.strftime (#6317)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/datetime.pyi

@@ -56,7 +56,7 @@ class date:
     @property
     def day(self) -> int: ...
     def ctime(self) -> str: ...
-    def strftime(self, fmt: str) -> str: ...
+    def strftime(self, __format: str) -> str: ...
     def __format__(self, __fmt: str) -> str: ...
     def isoformat(self) -> str: ...
     def timetuple(self) -> struct_time: ...
@@ -128,7 +128,7 @@ class time:
     if sys.version_info >= (3, 7):
         @classmethod
         def fromisoformat(cls: Type[_S], __time_string: str) -> _S: ...
-    def strftime(self, fmt: str) -> str: ...
+    def strftime(self, __format: str) -> str: ...
     def __format__(self, __fmt: str) -> str: ...
     def utcoffset(self) -> timedelta | None: ...
     def tzname(self) -> str | None: ...