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

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.54%. Comparing base (b1247d1) to head (a1e7a8e).
Report is 51 commits behind head on main.

Files with missing lines	Patch %	Lines
pandas_datareader/data.py	78.57%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            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     

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