BUG: Fix interval_range when start/periods or end/periods are specified with float start/end

[jschendel]

• closes BUG: interval_range is incorrect when start/periods or end/periods is specified with float start/end #21161
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Should be good for 0.23.1; the code changes only hit the interval_range function, which is outside the IntervalIndex class, so should be independent of any enhancements related to IntervalIndex that we might want to push to 0.24.0.

[codecov]

Codecov Report

Merging #21162 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #21162   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039

Flag	Coverage Δ
#multiple	90.23% <100%> (ø)	⬆️
#single	41.88% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/interval.py	93.14% <100%> (ø)	⬆️

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[jreback]

lgtm. looks like a conflict in whatsother. merge on fix. (note ok to add some blank lines in each of the bug fix sections to avoid future conflicts)