Fix failing builds: python 2.7 is failing on travis CI & TravisCI.org is about to stop service anyway

[mikofski]

Here's how it looks in the docs and README:

Follow the link: https://travis-ci.org/github/SunPower/PVMismatch, and you see it's the Python-2.7 build that fails. Has PVM officially dropped Py2?

And anyway, there's this alert from TravisCI.org:

So that means that builds and testing need to be migrated to Azure pipelines. We can use what pvlib has done to figure it out

[chetan201]

@mikofski I took a look and couldn't find any obvious reasons why Py27 should fail. If Azure pipelines is easy to migrate to, I also support the idea. What do you think about Github actions?
A cursory search landed me here - https://docs.github.com/en/free-pro-team@latest/actions/guides/about-continuous-integration

[kandersolar]

Looks like the dulwich dependency is the culprit. Per https://www.dulwich.io/docs/#supported-versions-of-python, they dropped support for python 2.7 starting in 0.20. The failed build installs 0.20.2 while the last passing build installed 0.19.16. So I guess a small update to requirements.txt would get the builds passing again.