Switch various commands to python3

[cboltz]

... instead of just 'python' which typically means python2.

[benadida]

if using pip and such, what's the point of this change?

[cboltz]

Well, if using pip ;-)

In my case, I create a RPM package (which then has dependencies on python3 and all the needed python3-$module packages).

The automatic dependency detection would add a Requires: python instead of Requires: python3 if it finds a #!/usr/bin/python line in a script like manage.py or .../oauth/rsa.py.
(The changes in INSTALL.md etc. are more cosmetic, but since using python3 is expected nowadays, I also updated them for consistency.)