mypy complains about slicing using strings

[Dr-Irv]

See discussion in #397

Having said that, I get the same error on this simple code with mypy, so it seems it is a mypy problem, as pyright allows this:

import pandas as pd
df = pd.DataFrame({"c1":[1,2,3,4], "c2":[10,20,30,40]}, index=["a", "b", "c" , "d"])

x =  df.loc["b":"c", :]

Seems like a long standing issue. See
python/mypy#2410
python/typeshed#8647

Originally posted by @Dr-Irv in #397 (comment)