MAINT: Add requirements

[bashtage]

Add requirements files to distribution

• closes requirements.txt missing from source 0.8.0 distribution in pypi #712

[codecov-io]

Codecov Report

Merging #714 into master will decrease coverage by 14.31%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #714       +/-   ##
===========================================
- Coverage   84.45%   70.14%   -14.32%     
===========================================
  Files          62       62               
  Lines        2753     2753               
  Branches      301      301               
===========================================
- Hits         2325     1931      -394     
- Misses        356      747      +391     
- Partials       72       75        +3

Impacted Files	Coverage Δ
pandas_datareader/av/forex.py	22.72% <0%> (-72.73%)	⬇️
pandas_datareader/tests/av/test_av_time_series.py	28.4% <0%> (-69.32%)	⬇️
pandas_datareader/av/quotes.py	26.47% <0%> (-64.71%)	⬇️
pandas_datareader/quandl.py	29.26% <0%> (-63.42%)	⬇️
pandas_datareader/av/time_series.py	32.43% <0%> (-62.17%)	⬇️
pandas_datareader/enigma.py	23.63% <0%> (-60%)	⬇️
pandas_datareader/tiingo.py	38.29% <0%> (-57.45%)	⬇️
pandas_datareader/tests/test_quandl.py	42.59% <0%> (-57.41%)	⬇️
pandas_datareader/tests/av/test_av_forex.py	50% <0%> (-50%)	⬇️
pandas_datareader/tests/av/test_av_quotes.py	50% <0%> (-46.88%)	⬇️
... and 8 more

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