lookaside: Drop EL < 6 support

This will make it easier to move away from md5 for the source tarballs.

It shouldn't cause any problem anyway, as Fedora runs this on EL 6 in
production, and EL 7 in staging.

roles/distgit/files/dist-git-upload.cgi

@@ -24,12 +24,8 @@ try:
 except ImportError:
     from email.MIMEText import MIMEText
 
-try:
-    import hashlib
-    md5_constructor = hashlib.md5
-except ImportError:
-    import md5
-    md5_constructor = md5.new
+import hashlib
+md5_constructor = hashlib.md5
 
 # Reading buffer size
 BUFFER_SIZE = 4096