pathencode: fix importing hashlib on Python 3

I'm curious why PyImport_Import is returning NULL, but
PyImport_ImportModule bypasses some of the regular import path,
probably including demandimport, which I suspect is the issue here.

Differential Revision: https://phab.mercurial-scm.org/D3587

mercurial/cext/pathencode.c

@@ -660,7 +660,7 static int sha1hash(char hash[20], const
 		if (name == NULL)
 			return -1;
 
-		hashlib = PyImport_Import(name);
+		hashlib = PyImport_ImportModule("hashlib");
 		Py_DECREF(name);
 
 		if (hashlib == NULL) {