PERF: use python int in RangeIndex.get_loc

[topper-123]

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:

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]

Codecov Report

Merging #26697 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            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)	⬆️
#single	41.93% <100%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/range.py	98.81% <100%> (+0.28%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

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[codecov]

Codecov Report

Merging #26697 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            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)	⬆️
#single	41.93% <100%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/range.py	98.81% <100%> (+0.28%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

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 649ad5c...cfbafe9. Read the comment docs.

[jreback]

thanks @topper-123