mypy: don't follow imports (#4368)

Author: Kludex

mypy.ini

@@ -17,4 +17,6 @@
   strict_equality = True
   strict_optional = True
   no_implicit_optional =  True
-  no_implicit_reexport = True
+  no_implicit_reexport = True
+  # https://mypy.readthedocs.io/en/stable/running_mypy.html#follow-imports
+  follow_imports = silent

opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.pyi

@@ -49,8 +49,8 @@ class BoundedList(Sequence[_T]):
     @overload
     def __getitem__(self, s: slice) -> Sequence[_T]: ...
     def __len__(self) -> int: ...
-    def append(self, item: _T): ...
-    def extend(self, seq: Sequence[_T]): ...
+    def append(self, item: _T) -> None: ...
+    def extend(self, seq: Sequence[_T]) -> None: ...
     @classmethod
     def from_seq(cls, maxlen: int, seq: Iterable[_T]) -> BoundedList[_T]: ...  # pylint: disable=undefined-variable