Update pymc version requirements.txt

[juanitorduz]

These are moving fast heh https://github.com/pymc-devs/pymc/releases/tag/v5.15.0

[maresb]

Hard pinning pymc to 5.15.0 is pretty harsh, no? I'm not following closely, but is there a particular reason?

[juanitorduz]

Hard pinning pymc to 5.15.0 is pretty harsh, no? I'm not following closely, but is there a particular reason?

Yes: see #162 (comment) (and initial scope of the PR)

[juanitorduz]

@maresb, do you have a suggestion on how to handle this in a way to see if a new release breaks something and, if so, restrict the version?

[maresb]

I'd recommend a < pin to the next untested version, e.g. <5.15. Otherwise you won't be compatible with patch releases. Also, more broadly this prevents people from using it with previous versions. Is it incompatible with 5.13? If so, then it should be something like <=5.14,<5.15.

More broadly, we should do more testing.

[juanitorduz]

ok! Let's see :) #164