add "upgreek" macro spec

[gamboz]

Packages like upgreek and newtxmath provide an "upright" version of greek letters.

[phfaist]

Hi, thanks for the contribution!

Before merging, I see some checks are failing. Does your code work on all python versions supported by pylatexenc (2.7+)? Also I noticed an extra test file specifically for the upgreek macros. Currently there aren't any good tests for pylatexenc.latexwalker's default latex definitions, and there should be some level of testing, but I'd like to avoid a manually maintained gigantic list that hard-codes all the definitions in order to test them. (I'd have in mind something like I use in the latexencode module, which automatically generates the table of symbols and compares it to a saved version of the table. I hope to be able to implement this soon.)

I'll come to editing and merging your pull request hopefully soon; this might take a moment as I'm pretty busy these days. Thanks again!

[gamboz]

Does your code work on all python versions supported by pylatexenc (2.7+)?

No, sorry, I did not test for this. The problem is probably in the test file, as it uses super().__init__()

I'll come to editing and merging your pull request hopefully soon...

thanks :)