makedep: Update interpreter directive to python3

The interpreter directive ("shebang") of makedep is updated to
`python3`, rather than the version-agnostic `python`.

Although we never invoke the shebang of the script, there are OS
environments out there which will object to any presence of a
versionless python.  PEP 394 also strongly recommends the adoption of
python3 as the executable name, regardless of Py2 support.

Author: marshallward

ac/makedep

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from __future__ import print_function