Break up requirements.txt into multiple requirements files

[jadchaar]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Break up requirements-dev.txt into 3 different files:

1. requirements.txt - main package dependencies specified in setup.py
2. requirements-docs.txt - doc dependencies
3. requirements-tests.txt - test dependencies

[codecov[bot]]

Codecov Report

Merging #1080 (467bf41) into master (74bc0a1) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1080   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2237      2237           
  Branches       439       439           
=========================================
  Hits          2237      2237           

Impacted Files	Coverage Δ
arrow/locales.py	100.00% <100.00%> (ø)

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