Fix pymssql import

[grihabor]

Description

Fix import error in luigi.contrib.mssqldb

Motivation and Context

I'm getting this error even though I have pymssql installed:

Traceback (most recent call last):
  File "/opt/luigi/lib/python3.10/site-packages/luigi/worker.py", line 434, in check_complete
    is_complete = check_complete_cached(task, completion_cache)
  File "/opt/luigi/lib/python3.10/site-packages/luigi/worker.py", line 419, in check_complete_cached
    is_complete = task.complete()
  File "/opt/luigi/lib/python3.10/site-packages/luigi/task.py", line 926, in complete
    return all(r.complete() for r in flatten(self.requires()))
  File "/opt/luigi/lib/python3.10/site-packages/luigi/task.py", line 926, in <genexpr>
    return all(r.complete() for r in flatten(self.requires()))
  File "/opt/luigi/lib/python3.10/site-packages/luigi/task.py", line 594, in complete
    return all(map(lambda output: output.exists(), outputs))
  File "/opt/luigi/lib/python3.10/site-packages/luigi/task.py", line 594, in <lambda>
    return all(map(lambda output: output.exists(), outputs))
  File "/opt/luigi/lib/python3.10/site-packages/luigi/contrib/mssqldb.py", line 104, in exists
    connection = self.connect()
  File "/opt/luigi/lib/python3.10/site-packages/luigi/contrib/mssqldb.py", line 123, in connect
    connection = _mssql.connect(user=self.user,
NameError: name '_mssql' is not defined

[dlstadther]

This file does not appear to have any unittests. In its place, can you confirm that after installing pymssql and instantiating content from this file (with your changes), it works as expected?

The changes look like they should be good, but I cannot easily confirm that all remaining imports and references are still correct.

[grihabor]

@dlstadther Yes, we're using this patch in production