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.