FIX: Installing IPython when building RTD docs

[TTsangSC]

Right now the online docs for line_profiler.ipython_extension are empty, probably because IPython is not installed in the build environment and thus said module is unavailable at build time. Considering the recent efforts on improving said module (#383) and inquiries thereon (#382), it makes sense for its docs to at least be publicly accessible.

Hence, this PR updates .readthedocs.yml requirements/docs.txt so that ReadTheDocs build the doc pages with IPython installed, and autodocs can thus (presumably and hopefully) build the page for the aforementioned submodule.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.77%. Comparing base (60e928f) to head (1b34c4d).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
- Coverage   87.56%   86.77%   -0.80%     
==========================================
  Files          18       20       +2     
  Lines        1641     1920     +279     
  Branches      348      365      +17     
==========================================
+ Hits         1437     1666     +229     
- Misses        149      187      +38     
- Partials       55       67      +12     

see 6 files with indirect coverage changes

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[Erotemic]

Probably would make more sense to put:

-r ipython.txt

in requirements/docs.txt