Use chevron Instead of pystache for Templating

[mcdonnnj]

🗣 Description

This PR replaces defunkt/pystache with noahmorrison/chevron for rendering mustache templates.

💭 Motivation and context

The pystache package's last release was 2014-05-12 while the chevron package's last release was 2021-01-02. This difference in continued development should be reason enough to change. In mustache/mustache.github.com#120 the recommended Python package for mustache templating was updated from pystache to chevron. As a last bonus:

"Chevron runs in less than half the time of pystache (Which is not even up to date on the spec). And in about 70% the time of Stache (A 'trimmed' version of mustache, also not spec compliant)."

This is a "test run" of transitioning to using chevron for any of our mustache templating. I think it will be beneficial to switch to a maintained package that is claimed to be substantially faster than our current package choice.

🧪 Testing

Automated tests pass successfully. But wait, there's more!

Generate rendered files with Python

>>> import chevron
>>> template_data = {
...   "conflict_file_list": ["foo.py", "bar.sh"],
...   "lineage_id": "skeleton",
...   "local_branch": "develop",
...   "metadata": 'lineage:metadata:{"slayed":true}',
...   "pr_branch_name": "lineage/skeleton",
...   "remote_branch": "HEAD",
...   "remote_url": "https://github.com/cisagov/skeleton-python-library.git",
...   "repo_name": "action-lineage",
...   "ssh_url": "git@github.com:cisagov/action-lineage.git",
... }
>>> with open("src/lineage/templates/clean_template.md") as f:
...   clean_template = f.read()
...
>>> with open("src/lineage/templates/conflict_template.md") as f:
...   conflict_template = f.read()
...
>>> clean_body = chevron.render(clean_template, template_data)
>>> conflict_body = chevron.render(conflict_template, template_data)
>>> with open("clean_rendered.md", "w") as f:
...   f.write(clean_body)
...
792
>>> with open("conflict_rendered.md", "w") as f:
...   f.write(conflict_body)
...
2397
>>> import pystache
>>> clean_body = pystache.render(clean_template, template_data)
>>> conflict_body = pystache.render(conflict_template, template_data)
>>> with open("clean_rendered_pystache.md", "w") as f:
...   f.write(clean_body)
...
792
>>> with open("conflict_rendered_pystache.md", "w") as f:
...   f.write(conflict_body)
...
2397
>>>

Compare the rendered files

$ diff clean_rendered.md clean_rendered_pystache.md
$ diff conflict_rendered.md conflict_rendered_pystache.md

✅ Checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

[dav3r]

Seems promising! 👍