Add missing Path.is_mount() introduced in Python 3.7 (#3566)

third_party/2/pathlib2.pyi

@@ -77,6 +77,8 @@ class Path(PurePath):
     def group(self) -> str: ...
     def is_dir(self) -> bool: ...
     def is_file(self) -> bool: ...
+    if sys.version_info >= (3, 7):
+        def is_mount(self) -> bool: ...
     def is_symlink(self) -> bool: ...
     def is_socket(self) -> bool: ...
     def is_fifo(self) -> bool: ...