[MRG] pin docutils to v0.16 to address doc building bug

[ctb]

This PR prevents docutils from upgrading to the latest release, v0.17, which causes some problems with Sphinx. See error message in #1437 for details.

Fixes #1437

[codecov]

Codecov Report

Merging #1444 (c31ba9a) into latest (c49a8d8) will increase coverage by 5.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1444      +/-   ##
==========================================
+ Coverage   89.44%   94.59%   +5.14%     
==========================================
  Files         123       96      -27     
  Lines       19105    15491    -3614     
  Branches     1471     1471              
==========================================
- Hits        17089    14654    -2435     
+ Misses       1784      605    -1179     
  Partials      232      232              

Flag	Coverage Δ
python	94.59% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/src/cmd.rs
src/core/tests/minhash.rs
src/core/src/sketch/minhash.rs
src/core/src/index/search.rs
src/core/src/encodings.rs
src/core/src/errors.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/bigsi.rs
src/core/src/index/linear.rs
src/core/src/from.rs
... and 17 more

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[ctb]

hi @luizirber @bluegenes @keyabarve @erikyoung85 I think this is ready for review!

In terms of review, the thing to pay attention to is the GitHub Actions tests ("checks") on this PR. If you look at other recent builds (e.g. #1443), you'll see that Python 3.8 tests fail on both Mac OS X and Linux, as does the readthedocs build; while in this build, they succeed. That's because here I'm pinning the docutils version to an older release (v.016) to avoid an error caused by the Apr 3, 2021 of docutils v0.17.

So for review of this, all you need to do is verify that the checks above all pass for this PR, and say "Looks good to me!"

[ctb]

Note that all checks have now passed! So this is an easy review 😁

[bluegenes]

lgtm :)

[ctb]

thanks!