PERF: speed up .ix by moving deprecation string out of __init__

[qwhelan]

While .ix is deprecated, the DeprecationWarning itself adds a decent fraction of overhead to calling it. Simply storing the warning string on the class (or global scope) yields a sizable speedup:

$ asv compare v0.24.0rc1 HEAD -s --only-changed --sort ratio
       before           after         ratio
     [fdc4db25]       [d0821a4c]
     <v0.24.0rc1^0>       <ix_speedup>
-      2.57±0.1ms      2.29±0.01ms     0.89  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.UInt64Index'>, 'nonunique_monotonic_inc')
-      2.69±0.2ms      2.39±0.02ms     0.89  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-         124±2μs          103±5μs     0.83  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.UInt64Index'>, 'unique_monotonic_inc')
-         149±4μs          121±4μs     0.81  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
-     2.43±0.08ms      1.96±0.05ms     0.81  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-        179±20μs          138±7μs     0.77  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-         506±6μs          384±6μs     0.76  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-        84.6±1μs         63.5±2μs     0.75  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
-        69.1±4μs         50.4±2μs     0.73  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-     1.22±0.02ms          728±2μs     0.60  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-        31.3±4μs       8.67±0.9μs     0.28  indexing.DataFrameStringIndexing.time_ix
-        16.5±1μs       2.67±0.3μs     0.16  indexing.MethodLookup.time_lookup_ix

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

[codecov]

Codecov Report

Merging #24747 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #24747   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48373    48373           
  Misses       3985     3985

Flag	Coverage Δ
#multiple	90.81% <100%> (ø)	⬆️
#single	43.07% <66.66%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexing.py	93.87% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #24747 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #24747   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48373    48373           
  Misses       3985     3985

Flag	Coverage Δ
#multiple	90.81% <100%> (ø)	⬆️
#single	43.07% <66.66%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexing.py	93.87% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 33f91d8...d0821a4. Read the comment docs.

[gfyoung]

That makes sense, given that it requires fewer looks to find the string. Surprising how the big change in performance is though...

[jreback]

ok looks fine. ping on green.

[jreback]

thanks @qwhelan