MAINT: Add tester

[bashtage]

Add tester to fully test install
Fix issues related to testing installed package
Update isort

closes #806

• closes Odd test variations based on download source #806
• 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 #809 into master will decrease coverage by 20.24%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           master     #809       +/-   ##
===========================================
- Coverage   84.62%   64.37%   -20.25%     
===========================================
  Files          62       62               
  Lines        2816     2824        +8     
  Branches      301      303        +2     
===========================================
- Hits         2383     1818      -565     
- Misses        360      932      +572     
- Partials       73       74        +1     

Impacted Files	Coverage Δ
pandas_datareader/conftest.py	58.33% <55.55%> (-1.67%)	⬇️
pandas_datareader/av/quotes.py	34.21% <100.00%> (ø)
pandas_datareader/compat/__init__.py	81.81% <100.00%> (-6.07%)	⬇️
pandas_datareader/data.py	53.50% <100.00%> (-25.48%)	⬇️
pandas_datareader/enigma.py	25.00% <100.00%> (ø)
pandas_datareader/io/sdmx.py	91.42% <100.00%> (ø)
pandas_datareader/moex.py	67.67% <100.00%> (ø)
pandas_datareader/tests/av/test_av_quotes.py	54.16% <100.00%> (-45.84%)	⬇️
pandas_datareader/tests/io/test_jsdmx.py	100.00% <100.00%> (ø)
pandas_datareader/tests/io/test_sdmx.py	100.00% <100.00%> (ø)
... and 43 more

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