API/REGR: Convert to float for index union

[TomAugspurger]

This restores the 0.24.x behavior of Index.union(other) between
Float and (U)Int indexes. These are again floating dtype.

left	right	output of left.union(right)
int	float	float64
int	uint	object
float	uint	float64

#26778 (comment)

Closes #26778

[jreback]

looks good. can you add a sub-section in whatsnew to explain the change.

+1 on the actual change semantics.

[codecov]

Codecov Report

Merging #27034 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #27034      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50799      +25     
==========================================
+ Hits        46711    46733      +22     
- Misses       4063     4066       +3

Flag	Coverage Δ
#multiple	90.63% <100%> (ø)	⬆️
#single	41.85% <46.15%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/numeric.py	97.6% <100%> (+0.26%)	⬆️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️

---

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[codecov]

Codecov Report

Merging #27034 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #27034      +/-   ##
==========================================
- Coverage   92.04%      92%   -0.04%     
==========================================
  Files         180      180              
  Lines       50714    50765      +51     
==========================================
+ Hits        46679    46706      +27     
- Misses       4035     4059      +24

Flag	Coverage Δ
#multiple	90.64% <100%> (-0.04%)	⬇️
#single	41.83% <58.33%> (-0.13%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/numeric.py	97.46% <100%> (+0.12%)	⬆️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/internals/managers.py	96.09% <0%> (-0.77%)	⬇️
pandas/core/internals/blocks.py	94.61% <0%> (-0.53%)	⬇️
pandas/core/panel.py	17.61% <0%> (-0.2%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️
pandas/core/indexing.py	93.48% <0%> (+0.18%)	⬆️

---

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[TomAugspurger]

looks good. can you add a sub-section in whatsnew to explain the change.

This is restoring the 0.24.2 behavior. I added a note in the user guide docs.

[jreback]

looks good. can you add a sub-section in whatsnew to explain the change.

This is restoring the 0.24.2 behavior. I added a note in the user guide docs.

That may be, but this should be up front about that in the whatsnew. IOW should be highly visible.

[TomAugspurger]

The CI failure from codecov is unrelated I think (cc @mroeschke. May just be a hiccup)

[jreback]

thanks @TomAugspurger