Avoid unconditional imports of pandas,pyarrow from rust

[timkpaine]

Right now we do some unconditional imports of pandas/pyarrow from rust:

perspective/rust/perspective-python/src/client/python.rs

Line 145 in 768ca40

let pyarrow = PyModule::import_bound(py, "pyarrow")?;

perspective/rust/perspective-python/src/client/python.rs

Line 199 in 768ca40

let pandas = PyModule::import_bound(py, "pandas")?;

Ideally, we wouldn't incur import costs just for this check, instead we can check if e.g. pandas in sys.modules. If it is, then the import is already paid for, if it isn't, then what we are receiving can't be a pandas Dataframe

[timkpaine]

xref: #2723

[texodus]

... if it isn't, then what we are receiving can't be a pandas Dataframe

What if I do this:

import sys
import pandas
import perspective

df = pandas.DataFrame({"a": [1,2,3]})
del sys.modules["pandas"]

perspective.table(df)

[timkpaine]

Deleting from sys modules is explicitly documented in the standard library docs as having the potential for causing problems, so no need to handle explicitly (I don't know why you would delete from sys modules unless you're trying to cause problems, this example is a degenerate case pretty clearly).

[texodus]

Fixed in #2754, released in v3.1.0