Raising DataError when passing DataFrame with int columns to Dataset

[hoxbro]

Fixes #5353

Note that this PR will break hvplot tests.

[codecov-commenter]

Codecov Report

Merging #5354 (8783487) into master (5ce1e1d) will not change coverage. The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #5354   +/-   ##
=======================================
  Coverage   88.07%   88.07%           
=======================================
  Files         301      301           
  Lines       61957    61957           
=======================================
  Hits        54567    54567           
  Misses       7390     7390           

Impacted Files	Coverage Δ
holoviews/core/data/pandas.py	93.40% <50.00%> (ø)

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