BUG: Fix dtypes of `pd.pivot_table`

[aresnow1]

What do these changes do?

Fix dtypes of pd.pivot_table.

Related issue number

Fixes #667

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass

[codecov[bot]]

Codecov Report

Merging #668 (6332243) into main (db72a87) will decrease coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #668      +/-   ##
==========================================
- Coverage   93.61%   93.51%   -0.11%     
==========================================
  Files        1025     1025              
  Lines       79735    79734       -1     
  Branches    16538    16538              
==========================================
- Hits        74645    74564      -81     
- Misses       3415     3498      +83     
+ Partials     1675     1672       -3     

Flag	Coverage Δ
unittests	93.40% <100.00%> (-0.11%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
python/xorbits/_mars/dataframe/base/pivot_table.py	99.31% <100.00%> (-0.01%)	:arrow_down:

... and 17 files with indirect coverage changes