[Python] Fix nogil declarations on libarrow_python.pxd

It seems some `nogil` declarations must be fixed for some of the libarrow_python APIs that actually take/return a `{}PyObject*{`}, or depend on Python C APIs.

**Reporter**: [Raúl Cumplido](https://issues.apache.org/jira/browse/ARROW-16196) / @raulcd

_{**Note**: *This issue was originally created as [ARROW-16196](https://issues.apache.org/jira/browse/ARROW-16196). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by inspecting libarrow_python.pxd, focusing on declarations involving PyObject* or Python C APIs and their nogil annotations. Done means the affected declarations no longer permit nogil where Python interaction makes that unsafe.

Written by the indexing model from the issue text.