Add 'dev-requirements.txt' to Pip Requirements

[moshemoshe137]

Add dev-requirements.txt as a recognized filename for the existing "Pip requirements" language.

Description

dev-requirements.txt is commonly used in Python projects in addition to the existing requirements-dev.txt filename. A GitHub code search returns 5.7k repos using this filename, including python/pythondotorg, pintrest/snappass, dropbox/sqlalchemy-stubs, Patreon/patreon-python, meta-pytorch/torchsnapshot, and oracle/oci-mlflow to name a few.

There are multiple Python tools that recommend a filename of dev-requirements.txt, including:

• pip-tools suggests creating a dev-requirements.txt for development dependencies. While not officially affiliated
  with pip, pip-tools has over 20 million downloads in the past month, according to online trackers.
• pipenv migration guide recommends exporting to dev-requirements.txt:

  poetry export -f requirements.txt --dev --output dev-requirements.txt

• deptry uses both dev-requirements.txt and requirements-dev.txt as default filenames for development dependencies.
• Veracode's SCA uses both dev-requirements.txt and requirements-dev.txt as default filenames for pip_requirements_file.

Since linguist already recognizes requirements-dev.txt and requirements.lock.txt for Pip Requirements, adding dev-requirements.txt seems like a consistent addition that will improve highlighting for thousands of repos.

I didn't open a separate issue for this small change... I apologize if that's a faux pas 😅

---

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=NOT+is%3Afork+path%3A%2F%5Edev-requirements.txt%24%2F&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/gevent/gevent/blob/master/dev-requirements.txt
    • Sample license(s):
      • MIT License
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

We need a sample too. Please add a sample and update OP to use the template to state where the sample came from and its source. You will need to use the original template. In this case the extension section should do.

[moshemoshe137]

Really sorry for messing up the PR template! I've added a sample file under the MIT license. I'm still learning, so I'm very grateful for your feedback and consideration 🙏 Please let me know if there is anything else I should adjust