update math and gzip stubs for Python 3.8 (#4710)

I ran stubtest and spotted 2 things missing in 3.8 stdlib

- added BadGzipFile exception for gzip:
https://docs.python.org/3/library/gzip.html#gzip.BadGzipFile

- added math.perm() method:
https://docs.python.org/3/library/math.html#math.perm

stdlib/3/gzip.pyi

@@ -43,6 +43,9 @@ class _PaddedFile:
     def seek(self, off: int) -> int: ...
     def seekable(self) -> bool: ...
 
+if sys.version_info >= (3, 8):
+    class BadGzipFile(OSError): ...
+
 class GzipFile(_compression.BaseStream):
     myfileobj: Optional[IO[bytes]]
     mode: str