Apply fix for count of group by query

[dwasyl]

This addresses the issues I raised in #12 and is simply reapply a fix proposed upstream at nattster@b095b20 and michiya@761a3a8 that didn't seem to make it into this fork.

[OskarPersson]

Looking at the Linux job on Travis, we see that this generates a new error when running the tests:

======================================================================
ERROR: test_alter_field_m2m (migrations.test_operations.OperationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/travis/build/ESSolutions/django-mssql-backend/sql_server/pyodbc/base.py", line 592, in execute
    return self.cursor.execute(sql, params)
pyodbc.ProgrammingError: ('42000', "[42000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Could not drop object 'test_alflmm_stable' because it is referenced by a FOREIGN KEY constraint. (3726) (SQLExecDirectW)")
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/tests/migrations/test_operations.py", line 31, in tearDown
    self.cleanup_test_tables()
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/tests/migrations/test_operations.py", line 40, in cleanup_test_tables
    'table': editor.quote_name(table_name),
  File "/home/travis/build/ESSolutions/django-mssql-backend/sql_server/pyodbc/schema.py", line 675, in execute
    cursor.execute(sql, params)
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/backends/utils.py", line 76, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/travis/build/ESSolutions/django-mssql-backend/django/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/home/travis/build/ESSolutions/django-mssql-backend/sql_server/pyodbc/base.py", line 592, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: ('42000', "[42000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Could not drop object 'test_alflmm_stable' because it is referenced by a FOREIGN KEY constraint. (3726) (SQLExecDirectW)")

[dwasyl]

@OskarPersson Any idea what in this would impact FK constraints? Admittedly, it's a bit beyond my internal knowledge of MSSQL/DB backends, but I don't mind plugging away if I have a general direction. On a working level, the patch seems to address a few issues and I haven't had any issue with M2M fields in my tests (although clearly there are issues somewhere).

[OskarPersson]

I have now tried to run this locally and it seems that test_alter_field_m2m is just a "flaky" test with this library, will have to look into that separately, i.e. we can ignore this error here.

Though I would prefer to see some change in the test results before merging this, ensuring that this works as it should.

[OskarPersson]

@dwasyl Can you add a test that fails without the change and succeeds after similar to the one in #15?