PERF: sort_index with an already monotonic MultiIndex

[lukemanley]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/v2.2.0.rst file if fixing a bug or adding a new feature.

import pandas as pd
import numpy as np

mi = pd.MultiIndex.from_product([np.arange(1000)] * 2)
df = pd.DataFrame(np.random.randn(len(mi)), index=mi)

%timeit df.sort_index()

# 100 ms ± 2.22 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  <- main
# 4.17 ms ± 62.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)  <- PR

[mroeschke]

Thanks @lukemanley