Add stubs for CGIHTTPServer (#3196)

This commit adds:
* Stubs for CGIHTTPServer in the Python 2 standard library, as requested in #1147.
* Stubs for six.moves.CGIHTTPServer in Python 2, as requested in #22.

stdlib/2/CGIHTTPServer.pyi

@@ -0,0 +1,8 @@
+# Stubs for CGIHTTPServer (Python 2.7)
+
+from typing import List
+import SimpleHTTPServer
+
+class CGIHTTPRequestHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
+    cgi_directories: List[str]
+    def do_POST(self) -> None: ...