Add genericpath stub (#2300)

To backport `os.path.commonpath` in mypy I needed to use genericpath. It seems unchanged since 3.4, and the `same*` functions were added in 3.4. (checked via comparing the `__all__`s of the source in 2.7, 3.4, and 3.7.)

stdlib/2and3/genericpath.pyi

@@ -0,0 +1,16 @@
+from typing import List
+import sys
+
+def exists(path: str) -> bool: ...
+def isfile(path: str) -> bool: ...
+def isdir(s: str) -> bool: ...
+def getsize(filename: str) -> int: ...
+def getmtime(filename: str) -> float: ...
+def getatime(filename: str) -> float: ...
+def getctime(filename: str) -> float: ...
+def commonprefix(m: List[str]) -> str: ...
+
+if sys.version_info >= (3, 4):
+    def samestat(s1: str, s2: str) -> int: ...
+    def samefile(f1: str, f2: str) -> int: ...
+    def sameopenfile(fp1: str, fp2: str) -> int: ...