arrow

GH-39313: [Python] Fix race condition in _pandas_api#_check_import (#39314)

### Rationale for this change

See:
```
    cdef inline bint _have_pandas_internal(self):
        if not self._tried_importing_pandas:
            self._check_import(raise_=False)
        return self._have_pandas
```

The method `_check_import`:
1) sets `_tried_importing_pandas` to true
2) does some things which take time...
3) sets `_have_pandas` to true (if we indeed do have pandas)

Suppose thread 1 calls `_have_pandas_internal`. If thread 1 is at step 2 while thread 2 calls `_have_pandas_internal`, `_have_pandas_internal` may incorrectly return False for thread 2 as thread 1 has set `_tried_importing_pandas` to true, but has not yet (but will) set `_have_pandas` to True. `_have_pandas_internal` will return True for thread 1.

After my fix, `_have_pandas_internal` will not return an incorrect value in the scenario described above. It would instead result in a redundant, but (I believe) harmless, invocation of `_check_import`.

### What changes are included in this PR?

Changes ordering of "trying to import pandas" and "recording that pandas import has been tried"

### Are these changes tested?
yes, see test committed

### Are there any user-facing changes?

This PR resolves a user-facing race condition https://github.com/apache/arrow/issues/39313
* Closes: #39313

Lead-authored-by: Thomas Jarosz <thomas.jarosz@c3.ai>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>