Use int instead of numpy.int

[sroet]

A test on a downstream project was showing Deprecation warnings from this code:

/home/sroet/miniconda3/envs/python3/lib/python3.9/site-packages/pyemma/coordinates/data/featurization/featurizer.py:240: DeprecationWarning: `np.int` is a deprecated alias for the builtin `int`. To silence this warning, use `int` by itself. Doing this will not modify any behavior and is safe. When replacing `np.int`, you may wish to use e.g. `np.int64` or `np.int32` to specify the precision. If you wish to review your current use, check the release note link for additional information.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    pair_inds = np.array(pair_inds).astype(dtype=np.int, casting='safe')

This PR solves that warning with the most common solution (just use the unaliased int)

• [NA] Make sure to include one or more tests for your change
• Add yourself to AUTHORS
• Add a new entry to the doc/source/CHANGELOG (choose any open position to avoid merge conflicts with other PRs).
  Decide whether your change is a fix or a new feature.

[sroet]

force pushed to rebase on top off #1505

[clonker]

Also looks good to me, thank you for your contributions! 🎉

[codecov]

Codecov Report

Merging #1504 (652d37c) into devel (366ce03) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            devel    #1504      +/-   ##
==========================================
+ Coverage   92.02%   92.03%   +0.01%     
==========================================
  Files         229      229              
  Lines       26804    26804              
==========================================
+ Hits        24666    24670       +4     
+ Misses       2138     2134       -4     

Impacted Files	Coverage Δ
...yemma/coordinates/data/featurization/featurizer.py	89.73% <100.00%> (ø)
pyemma/coordinates/data/util/traj_info_backends.py	93.72% <0.00%> (+1.79%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 366ce03...652d37c. Read the comment docs.