MAINT: Remove pd.Panel

[bashtage]

Remove final use of pd.Panel

[codecov]

Codecov Report

Merging #218 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #218      +/-   ##
=========================================
- Coverage   99.72%   99.7%   -0.02%     
=========================================
  Files          83      83              
  Lines       14443   14454      +11     
  Branches     1419    1421       +2     
=========================================
+ Hits        14403   14412       +9     
- Misses          9      10       +1     
- Partials       31      32       +1

Impacted Files	Coverage Δ
linearmodels/tests/test_utility.py	100% <100%> (ø)	⬆️
linearmodels/panel/utility.py	98.5% <0%> (-1%)	⬇️

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