PERF: use python int in RangeIndex.get_loc
Follow-up to #26565. I didn't ensure a python int in ``RangeIndex.get_loc`` in the original PR, causing a slow performance when using a key with a wrong type:

```python
In [1]: rng =  pd.RangeIndex(1_000_000)
In [2]: %timeit rng.get_loc(np.int64(900_000))
156 ms ± 1.03 ms per loop  # master
478 ns ± 6.3 ns per loop # this PR
```

Also some cleanups in ``__getitem__``.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=h1) Report
> Merging [#26697](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/649ad5c91aabb95d7f200eec84b82cae1459fc65?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26697/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26697      +/-   ##
==========================================
- Coverage   91.88%   91.88%   -0.01%     
==========================================
  Files         174      174              
  Lines       50701    50699       -2     
==========================================
- Hits        46588    46583       -5     
- Misses       4113     4116       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <100%> (ø)` | :arrow_up: |
| #single | `41.93% <100%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `98.81% <100%> (+0.28%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26697?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/26697?src=pr&el=footer). Last update [649ad5c...cfbafe9](https://codecov.io/gh/pandas-dev/pandas/pull/26697?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/26697?src=pr&el=h1) Report
> Merging [#26697](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/649ad5c91aabb95d7f200eec84b82cae1459fc65?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26697/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26697      +/-   ##
==========================================
- Coverage   91.88%   91.88%   -0.01%     
==========================================
  Files         174      174              
  Lines       50701    50699       -2     
==========================================
- Hits        46588    46583       -5     
- Misses       4113     4116       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <100%> (ø)` | :arrow_up: |
| #single | `41.93% <100%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `98.81% <100%> (+0.28%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26697/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26697?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/26697?src=pr&el=footer). Last update [649ad5c...cfbafe9](https://codecov.io/gh/pandas-dev/pandas/pull/26697?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 