Update README re: Python 3

[molpopgen]

The fence for executable code doesn't work on systems where python is not an alias for python3. This change suggests how to fix.

[gpoore]

Thanks for the suggestion! I'll make sure that is clarified in the README for the next release.

I've actually thought about using shutil.which() to automatically swap the executable from python to python3 when python3 exists...there must be a version of Python 3 installed if text2qti is running. Do you have any thoughts about that?

[molpopgen]

It may be a bit more straightforward to internally check which Python interpreter is being run via sys:

In [7]: sys.version_info                                                                     
Out[7]: sys.version_info(major=3, minor=8, micro=2, releaselevel='final', serial=0)
In [8]: sys.version_info.major                                                               
Out[8]: 3

You can use the major version to decide to call python3.

[molpopgen]

there must be a version of Python 3 installed if text2qti is running. Do you have any thoughts about that

I just noticed that your README does say that 3.6 or later is required. In that case, I think it is harmless to simply say that the code fence should say {.python3 .run}. In a conda environment with Python 3, python is just a symlink/alias to python3. I imagine that python3 has to exist as an executable because 2/3 co-installations are going to exist for some time.

[gpoore]

I've added a note about using .python3 in the README. I've also modified the handling of .python, so that it uses python3 on systems where python is equivalent to python2 (those systems now require an explicit .python2 to use python2). And I've added the option of specifying an executable path for code blocks via executable=<executable> to provide even more customization.