Remove magic import from sh module

Since the magic import "from sh import pip"
is harder to debug and has problems on some
systems, it is now substituted by a conventional
pip-command object creation.

mayan/apps/installation/classes.py

@@ -7,7 +7,7 @@ import os
 import sh
 
 try:
-    from sh import pip
+    pip = sh.Command('pip')
     PIP = True
 except sh.CommandNotFound:
     PIP = False