Handle pandas urlencode removal.

[cottrell]

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• passes black --check pandas_datareader
• added entry to docs/source/whatsnew/vLATEST.txt

[codecov-commenter]

Codecov Report

Merging #793 into master will decrease coverage by 13.14%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #793       +/-   ##
===========================================
- Coverage   83.04%   69.89%   -13.15%     
===========================================
  Files          62       62               
  Lines        2860     2860               
  Branches      309      309               
===========================================
- Hits         2375     1999      -376     
- Misses        409      780      +371     
- Partials       76       81        +5     

Impacted Files	Coverage Δ
pandas_datareader/base.py	88.97% <100.00%> (+2.94%)	⬆️
pandas_datareader/iex/__init__.py	73.17% <100.00%> (ø)
pandas_datareader/av/forex.py	27.65% <0.00%> (-68.09%)	⬇️
pandas_datareader/quandl.py	30.95% <0.00%> (-61.91%)	⬇️
pandas_datareader/tests/test_quandl.py	42.59% <0.00%> (-57.41%)	⬇️
pandas_datareader/av/time_series.py	41.86% <0.00%> (-53.49%)	⬇️
pandas_datareader/tiingo.py	42.57% <0.00%> (-53.47%)	⬇️
pandas_datareader/tests/av/test_av_time_series.py	30.76% <0.00%> (-51.65%)	⬇️
pandas_datareader/tests/av/test_av_forex.py	58.06% <0.00%> (-41.94%)	⬇️
pandas_datareader/enigma.py	23.63% <0.00%> (-41.82%)	⬇️
... and 13 more

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