Build docs on modern Sphinx

[stigrj]

Hopefully fixing readthedocs rendering

[codecov]

Codecov Report

Merging #143 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   67.11%   67.11%           
=======================================
  Files         171      171           
  Lines       12138    12138           
=======================================
  Hits         8146     8146           
  Misses       3992     3992           

Impacted Files	Coverage Δ
src/treebuilders/add.cpp	100.00% <ø> (ø)
src/treebuilders/apply.cpp	70.00% <ø> (ø)
src/treebuilders/grid.cpp	72.00% <ø> (ø)
src/treebuilders/map.cpp	100.00% <ø> (ø)
src/treebuilders/multiply.cpp	79.31% <ø> (ø)
src/utils/mpi_utils.cpp	0.00% <ø> (ø)

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[MinazoBot]

	1 Warning
⚠️	There are code changes, but no corresponding tests. Please include tests if this PR introduces any modifications in behavior.

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