Fix/warnings

[michalk8]

IMPORTANT: Please search among the Pull requests before creating one.

In preparation to #469, I've updated requirements.txt to include all packages we use.
Currently Pillow and tiffffile have no pins (were previously pinned by scikit-image).

Description

Remove various warnings.

How has this been tested?

N/A

Closes

closes #466

[codecov]

Codecov Report

Merging #489 (19dbfe4) into master (99bb43a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   77.98%   77.99%   +0.01%     
==========================================
  Files          31       31              
  Lines        4343     4345       +2     
  Branches      794      795       +1     
==========================================
+ Hits         3387     3389       +2     
  Misses        734      734              
  Partials      222      222              

Impacted Files	Coverage Δ
squidpy/gr/_ligrec.py	79.64% <100.00%> (ø)
squidpy/gr/_nhood.py	82.87% <100.00%> (ø)
squidpy/im/_container.py	86.78% <100.00%> (+0.04%)	⬆️
squidpy/im/_feature_mixin.py	87.42% <100.00%> (ø)
squidpy/im/_process.py	83.82% <100.00%> (ø)
squidpy/pl/_graph.py	82.48% <100.00%> (ø)
squidpy/pl/_ligrec.py	90.72% <100.00%> (ø)