Add substr tests

[ayushdg]

As discussed in #258 it seems like SUBSTR is indeed supported and the error was most likely due to case sensitivity.

This PR adds a couple of test cases for SUBSTR since only SUBSTRING was covered.

Also a minor change that replaces the use of now deprecated distutils.version.LooseVersion with packaging.version.Version

[charlesbluca]

LGTM; thanks for adding these tests @ayushdg 😄

Test failures seem to be because of the breakage in dask-ml

[codecov-commenter]

Codecov Report

Merging #372 (420ffa9) into main (408f976) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
- Coverage   95.57%   95.46%   -0.11%     
==========================================
  Files          67       67              
  Lines        2935     2932       -3     
  Branches      547      546       -1     
==========================================
- Hits         2805     2799       -6     
- Misses         79       83       +4     
+ Partials       51       50       -1     

Impacted Files	Coverage Δ
dask_sql/_compat.py	100.00% <100.00%> (ø)
dask_sql/physical/rel/custom/show_models.py	53.33% <0.00%> (-26.67%)	⬇️
dask_sql/physical/utils/sort.py	93.10% <0.00%> (+2.47%)	⬆️

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