BUG: is_dtype_equal(dtype, "string[pyarrow]") raises if pyarrow not installed

[jbrockmendel]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=68369&view=logs&j=915dbb23-6529-54ae-059d-ec0b59ba10e8&t=86b091c2-17fe-52fe-2454-5f56a371e155&l=2889

Issue Description

Looks like it tries to construct the dtype and raises an ImportError

Expected Behavior

False

Installed Versions

Replace this line with the output of pd.show_versions()