Improve detection of supported standards

[GPMueller]

See also https://stackoverflow.com/a/49367391/4069571

• The test should be done without a file
• The test should check a few known numbers (89, 03, 11, 14, 17, 20)
• Numbers should be padded with zeros (currently 3 is checked, but 03 is not)

[GPMueller]

Implemented padding with 61db826.

[GPMueller]

Unfortunately the suggested solution to not use a file does not work so trivially with Python2...

[NOhs]

Quoting from the Readme

One compiler (Clang), one build system (written in Python)

I don't read: "One build system that supports every Python version there is, because we have to capture every possible scenario"

Therefore, dropping Python2 support seems to be a valid option to me. On Windows, most ppl have Python3 anyways via Anaconda I would guess. On modern Linux, python3 can be easily installed via the respective package manager. On Mac OSX I would also assume that python3 can be easily installed with homebrew or so. Therefore, I do not think dropping Python2 support is so dramatic.

[GPMueller]

Only supporting Python3 from PR #23 onwards, where the proper detection of c++ dialects is implemented.

[GPMueller]

Merged with a7bbc88.