No longer use undocumented module "sre_constants"

[serhiy-storchaka]

Closes #378.

[serhiy-storchaka]

It is my first contribution so I do not know some things.

Should I add an entry in the CHANGES file?

What about pyparsing_archive.py? Should I change it too or it is some historical artifact?

[ptmcg]

Very nice PR, thanks, I can merge it as-is.
No, do not update pyparsing_archive.py - it is there as a reference artifact that pre-dates the refactoring into multiple files.
I'll take care of the CHANGES blurb before the next release.
(How did the helpers.py code even work, since there is no import of sre_constants? Maybe it gets pulled in when importing re.)

For future reference:
It is good to test changes like this going back to the oldest supported Python version (currently 3.6.8). Not necessary for this case, I just confirmed that re.error works on Py3.6.8.

I'll add notes to the CONTRIBUTING file with these details.

[serhiy-storchaka]

re.error is available since Python 2.0.

helpers.py get sre_constants from from .core import *.