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