blurb should error out if run with a Python interpreter which is too old

[rodrigc]

The blurb command relies on Python 3.5+ in order to run.
If run with an older Python interpreter, weird errors happen such as:

   File "/usr/bin/blurb", line 123
     def textwrap_body(body, *, subsequent_indent=''):
                              ^
SyntaxError: invalid syntax

For newcomers to Python, this is hard to figure out.

blurb should error out if the Python version is too old.
I recommend that the code check the value of sys.version_info and
error out with a message indicating that the Python version is too old.

[rodrigc]

@caavery ran into this issue

[zware]

The problem is that the SyntaxError is thrown before we have a chance to check sys.version_info. There are any number of more or less horrible hacks that can be added to make this check, but I'm not sure whether any of them are worth it.

/cc: @larryhastings

[larryhastings]

They're not. blurb should be installed using "pip", and that will fail if you use python < 3.5. This is a rare circumstance, and as Zachary points out, painful to fix.

[CuriousLearner]

I think this needs to be handled. I faced the same issue and the blurb gets successfully installed with Python < 3.5.

[vstinner]

We can add a simple test on sys.version to prevent any attempt to install blurb on Python older than 3.5.

[rodrigc]

@Haypo @CuriousLearner See: #174

pip falls back to installing a version of blurb which does not fail if installed on Python 2.7.