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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=h1) Report
> Merging [#24747](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/33f91d8f9f2e84f2b5f3ac3f0481b691c977c427?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24747/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `43.07% <66.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24747/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.87% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=footer). Last update [33f91d8...d0821a4](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=h1) Report
> Merging [#24747](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/33f91d8f9f2e84f2b5f3ac3f0481b691c977c427?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24747/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `43.07% <66.66%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24747/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.87% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=footer). Last update [33f91d8...d0821a4](https://codecov.io/gh/pandas-dev/pandas/pull/24747?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 That makes sense, given that it requires fewer looks to find the string.  Surprising how the big change in performance is though... ok looks fine. ping on green. thanks @qwhelan 