[10sp] Make test (and other python code) Python3 compatible

[kostja]

Python3 is the next major version and some Linux distros (e.g. Linux Arch) are already use it as a default. It makes sense to make the code python3 compatible. In theory conversion should be simple especially if you use tools like py2to3.

Moved from tarantool/tarantool#56

[anatol]

Python2 is EOL. Python3 is a default version at major OS distributions. Could you please prioritize this effort and make tarantool project python2-free?

[ligurio]

I would try to decompose ticket to subtasks:

• remove relative module imports
• fix module imports for modules changed in Python 3 (for example ConfigParser)
• fix printing to make it compatible with 2 and 3 versions
• support Python 3 in Tarantool tests (Support Python 3 in tests tarantool#5538)