BUG: fix pivot table dtypes

[Hank0626]

What do these changes do?

Related issue number

Fixes #xxxx

Check code requirements

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

[codecov[bot]]

Codecov Report

Merging #669 (894ef47) into main (db72a87) will decrease coverage by 18.61%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             main     #669       +/-   ##
===========================================
- Coverage   93.61%   75.01%   -18.61%     
===========================================
  Files        1025     1025               
  Lines       79735    79739        +4     
  Branches    16538    16539        +1     
===========================================
- Hits        74645    59816    -14829     
- Misses       3415    17809    +14394     
- Partials     1675     2114      +439     

Flag	Coverage Δ
unittests	74.89% <0.00%> (-18.62%)	: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	28.47% <0.00%> (-70.85%)	:arrow_down:

... and 269 files with indirect coverage changes

[aresnow1]

duplicated with #668