tox docs: Change deps path to use requirements.txt

[loqs]

RTD-requirements.txt was removed in 8d81f96.

What do these changes do?

Are there changes in behavior for the user?

No

Related issue number

Related #544.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): docs
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.84%. Comparing base (a55d3de) to head (421c73d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
- Coverage   97.86%   97.84%   -0.02%     
==========================================
  Files          23       23              
  Lines        5701     5701              
  Branches      766      766              
==========================================
- Hits         5579     5578       -1     
- Misses         77       79       +2     
+ Partials       45       44       -1     

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