Bugfix/1760 Bad type hint for argument unique for DataFrameSchema

[Jarek-Rolski]

Problem description:

• init sets _unique attribute
• class DataFrameSchema defines unique property, getter returns _unique attribute, setter modifies string value into list of strings and saves to _unique attribute
• other package functions use unique property

Because of above, unique property setter is never invoked. Thus string value is never turned into list of strings as I believe was intended.

I added code turning string into list of string to __init__ function. I've also found same issue in dataframe and pyspark modules.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.23%. Comparing base (812b2a8) to head (c0e68d9).
Report is 150 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1817      +/-   ##
==========================================
- Coverage   94.28%   93.23%   -1.05%     
==========================================
  Files          91      120      +29     
  Lines        7013     9155    +2142     
==========================================
+ Hits         6612     8536    +1924     
- Misses        401      619     +218     

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[cosmicBboy]

thanks @Jarek-Rolski, and congrats on your first PR to pandera! 🚀