test: added tests for polygon

[ErikBjare]

Works fine, just need to fix so the tests have the assets they need.

[codecov]

Codecov Report

Merging #267 (8fad544) into master (f74d23a) will increase coverage by 3.23%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   78.73%   81.96%   +3.23%     
==========================================
  Files          10       10              
  Lines         804      815      +11     
==========================================
+ Hits          633      668      +35     
+ Misses        171      147      -24     

Impacted Files	Coverage Δ
uniswap/tokens.py	70.00% <28.57%> (-15.72%)	⬇️
uniswap/cli.py	91.66% <66.66%> (-1.30%)	⬇️
uniswap/types.py	100.00% <0.00%> (ø)
uniswap/util.py	92.30% <0.00%> (+0.20%)	⬆️
uniswap/decorators.py	94.11% <0.00%> (+0.78%)	⬆️
uniswap/uniswap.py	79.09% <0.00%> (+4.70%)	⬆️

---

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 f74d23a...8fad544. Read the comment docs.