Drop support for Python 3.5 (#4675)

Python 3.5 EOL was on 2020-09-30.

stdlib/3/enum.pyi

@@ -29,13 +29,12 @@ class Enum(metaclass=EnumMeta):
     _value2member_map_: Dict[int, Enum]  # undocumented
     if sys.version_info >= (3, 7):
         _ignore_: Union[str, List[str]]
-    if sys.version_info >= (3, 6):
-        _order_: str
-        __order__: str
-        @classmethod
-        def _missing_(cls, value: object) -> Any: ...
-        @staticmethod
-        def _generate_next_value_(name: str, start: int, count: int, last_values: List[Any]) -> Any: ...
+    _order_: str
+    __order__: str
+    @classmethod
+    def _missing_(cls, value: object) -> Any: ...
+    @staticmethod
+    def _generate_next_value_(name: str, start: int, count: int, last_values: List[Any]) -> Any: ...
     def __new__(cls: Type[_T], value: object) -> _T: ...
     def __repr__(self) -> str: ...
     def __str__(self) -> str: ...
@@ -49,25 +48,26 @@ class IntEnum(int, Enum):
 
 def unique(enumeration: _S) -> _S: ...
 
-if sys.version_info >= (3, 6):
-    _auto_null: Any
+_auto_null: Any
 
-    # subclassing IntFlag so it picks up all implemented base functions, best modeling behavior of enum.auto()
-    class auto(IntFlag):
-        value: Any
-    class Flag(Enum):
-        def __contains__(self: _T, other: _T) -> bool: ...
-        def __repr__(self) -> str: ...
-        def __str__(self) -> str: ...
-        def __bool__(self) -> bool: ...
-        def __or__(self: _T, other: _T) -> _T: ...
-        def __and__(self: _T, other: _T) -> _T: ...
-        def __xor__(self: _T, other: _T) -> _T: ...
-        def __invert__(self: _T) -> _T: ...
-    class IntFlag(int, Flag):
-        def __or__(self: _T, other: Union[int, _T]) -> _T: ...
-        def __and__(self: _T, other: Union[int, _T]) -> _T: ...
-        def __xor__(self: _T, other: Union[int, _T]) -> _T: ...
-        __ror__ = __or__
-        __rand__ = __and__
-        __rxor__ = __xor__
+# subclassing IntFlag so it picks up all implemented base functions, best modeling behavior of enum.auto()
+class auto(IntFlag):
+    value: Any
+
+class Flag(Enum):
+    def __contains__(self: _T, other: _T) -> bool: ...
+    def __repr__(self) -> str: ...
+    def __str__(self) -> str: ...
+    def __bool__(self) -> bool: ...
+    def __or__(self: _T, other: _T) -> _T: ...
+    def __and__(self: _T, other: _T) -> _T: ...
+    def __xor__(self: _T, other: _T) -> _T: ...
+    def __invert__(self: _T) -> _T: ...
+
+class IntFlag(int, Flag):
+    def __or__(self: _T, other: Union[int, _T]) -> _T: ...
+    def __and__(self: _T, other: Union[int, _T]) -> _T: ...
+    def __xor__(self: _T, other: Union[int, _T]) -> _T: ...
+    __ror__ = __or__
+    __rand__ = __and__
+    __rxor__ = __xor__