[SPARK-19561] [PYTHON] cast TimestampType.toInternal output to long

## What changes were proposed in this pull request?

Cast the output of `TimestampType.toInternal` to long to allow for proper Timestamp creation in DataFrames near the epoch.

## How was this patch tested?

Added a new test that fails without the change.

dongjoon-hyun davies Mind taking a look?

The contribution is my original work and I license the work to the project under the project’s open source license.

Author: Jason White <jason.white@shopify.com>

Closes #16896 from JasonMWhite/SPARK-19561.