Bump Sphinx up to 3.4.0

[crcrpar]

related #1614

[codecov-io]

Codecov Report

Merging #2127 (0f746ee) into master (f65e66b) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2127      +/-   ##
==========================================
- Coverage   92.29%   92.27%   -0.03%     
==========================================
  Files         123      123              
  Lines        9972     9972              
==========================================
- Hits         9204     9202       -2     
- Misses        768      770       +2     

Impacted Files	Coverage Δ
optuna/integration/botorch.py	96.88% <0.00%> (-0.89%)	⬇️

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[HideakiImamura]

Thanks for the PR! I confirmed that CI downloaded the sphinx 3.4.0 and succeeded. LGTM.

Collecting sphinx
  Downloading Sphinx-3.4.0-py3-none-any.whl (2.9 MB)

[HideakiImamura]

@hvy Could you review this PR?

[hvy]

Thanks a bunch, LGTM!

Worked well on RTD as well (https://hvy-optuna.readthedocs.io/en/bump-sphinx-340-hvy/).