Feature/enable custom datareader

[begoat]

• closes stooq.com cannot get data related to USDJPY #895
• 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

[bashtage]

What does this bring that end users couldn't accomplish with subclassing?

[begoat]

What does this bring that end users couldn't accomplish with subclassing?

Hi @bashtage

I came across this problem. #895
I cannot figure out one way to request https://stooq.com/q/d/l/?s=USDJPY

[codecov-commenter]

Codecov Report

Merging #896 (a1e7a8e) into main (b1247d1) will decrease coverage by 1.60%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
- Coverage   73.15%   71.54%   -1.61%     
==========================================
  Files          65       65              
  Lines        2932     2963      +31     
  Branches      315      316       +1     
==========================================
- Hits         2145     2120      -25     
- Misses        705      760      +55     
- Partials       82       83       +1     

Impacted Files	Coverage Δ
pandas_datareader/__init__.py	40.90% <ø> (ø)
pandas_datareader/data.py	66.66% <78.57%> (+1.69%)	⬆️
pandas_datareader/tests/test_data.py	100.00% <100.00%> (ø)
pandas_datareader/tests/test_eurostat.py	40.47% <0.00%> (-59.53%)	⬇️
pandas_datareader/eurostat.py	61.29% <0.00%> (-25.81%)	⬇️
pandas_datareader/io/sdmx.py	80.00% <0.00%> (-11.43%)	⬇️
pandas_datareader/io/util.py	66.66% <0.00%> (-11.12%)	⬇️
pandas_datareader/tests/test_iex.py	94.44% <0.00%> (-5.56%)	⬇️
pandas_datareader/compat/__init__.py	86.84% <0.00%> (-2.64%)	⬇️

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