Fix usage of pandas.io.common.get_filepath_or_buffer

[kpj]

It seems like pandas (I am using v1.2.1) renamed pandas.io.common.get_filepath_or_buffer to pandas.io.common._get_filepath_or_buffer and changed its signature.
This makes, e.g., code like this crash:

import pandas_datareader as pdr
pdr.data.DataReader('hlth_hlye', 'eurostat')

This PR makes pandas_datareader.compat.get_filepath_or_buffer use the new interface.

[codecov-commenter]

Codecov Report

Merging #860 (d5903db) into master (90f155a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #860      +/-   ##
==========================================
+ Coverage   64.03%   64.05%   +0.01%     
==========================================
  Files          65       65              
  Lines        2906     2907       +1     
  Branches      311      311              
==========================================
+ Hits         1861     1862       +1     
  Misses        969      969              
  Partials       76       76              

Impacted Files	Coverage Δ
pandas_datareader/compat/__init__.py	82.35% <100.00%> (+0.53%)	⬆️

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