Split and restore correct Python 2 version of cgi.pyi (#5064)

* Split cgi.pyi into Py2 and 3 versions

* Restore correct Python 2 version of cgi.pyi

* Remove unnecessary version checks in cgi.pyi

* Use collections.abc

* Use List in Python 2 stub

stdlib/VERSIONS

@@ -54,7 +54,7 @@ builtins: 3.6
 bz2: 2.7
 cProfile: 2.7
 calendar: 2.7
-cgi: 2.7
+cgi: 3.6
 cgitb: 2.7
 chunk: 2.7
 cmath: 2.7