Rdkit compat

[hadim]

Checklist:

• [x] Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• [x] Update the API documentation is a new function is added or an existing one is deleted.
• [x] Added a news entry.
  • copy news/TEMPLATE.rst to news/my-feature-or-branch.rst) and edit it.

---

Make dm.descriptors.n_heavy_atoms compatible with rdkit 2021.03.

[codecov[bot]]

Codecov Report

Merging #155 (0b9312d) into main (9255d03) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #155   +/-   ##
=======================================
  Coverage   90.62%   90.63%           
=======================================
  Files          46       46           
  Lines        3297     3300    +3     
=======================================
+ Hits         2988     2991    +3     
  Misses        309      309           

Flag	Coverage Δ
unittests	90.63% <100.00%> (+<0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
datamol/descriptors/descriptors.py	92.20% <100.00%> (+0.31%)	:arrow_up:

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