Fix and test TimedeltaIndex.__rfloordiv__ bug

[jbrockmendel]

I thought we were ready to get rid of _TimeOp, but doing so turned up a couple of new bugs.

Setup:

scalar_td = pd.Timedelta('10m7s')
tdi = TimedeltaIndex(['00:05:03', '00:05:03', pd.NaT], freq=None)

Before:

>>> tdi.__rfloordiv__(scalar_td)
Float64Index([0.0, 0.0, nan], dtype='float64')

After:

>>> tdi.__rfloordiv__(scalar_td)
Float64Index([2.0, 2.0, nan], dtype='float64')

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19125 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19125      +/-   ##
==========================================
+ Coverage   91.51%   91.53%   +0.02%     
==========================================
  Files         148      148              
  Lines       48777    48780       +3     
==========================================
+ Hits        44636    44651      +15     
+ Misses       4141     4129      -12

Flag	Coverage Δ
#multiple	89.9% <100%> (+0.02%)	⬆️
#single	41.59% <25%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/base.py	96.45% <100%> (ø)	⬆️
pandas/core/indexes/timedeltas.py	90.63% <100%> (+0.05%)	⬆️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jreback]

thanks!