workaround a bug with the ast unparse package and Python 3.8

[shinnar]

No description provided.

[shinnar]

This is a workaround for simonpercivall/astunparse#43 which is causing problems for us on Python 3.8

[codecov]

Codecov Report

Merging #738 (e3bd344) into master (fde33e6) will decrease coverage by 0.02%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
- Coverage   82.05%   82.03%   -0.03%     
==========================================
  Files         337      337              
  Lines       17051    17063      +12     
==========================================
+ Hits        13992    13997       +5     
- Misses       3059     3066       +7     

Impacted Files	Coverage Δ
lale/expressions.py	82.89% <72.72%> (-0.91%)	⬇️
lale/lib/sklearn/stacking_utils.py	10.52% <0.00%> (-1.24%)	⬇️
lale/grammar.py	77.19% <0.00%> (-0.88%)	⬇️
lale/operators.py	83.92% <0.00%> (-0.05%)	⬇️

---

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 fde33e6...e3bd344. Read the comment docs.