[SPARK-23319][TESTS][FOLLOWUP] Fix a test for Python 3 without pandas.

## What changes were proposed in this pull request?

This is a followup pr of apache#20487.

When importing module but it doesn't exists, the error message is slightly different between Python 2 and 3.

E.g., in Python 2:

```
No module named pandas
```

in Python 3:

```
No module named 'pandas'
```

So, one test to check an import error fails in Python 3 without pandas.

This pr fixes it.

## How was this patch tested?

Tested manually in my local environment.

Author: Takuya UESHIN <[email protected]>

Closes apache#20538 from ueshin/issues/SPARK-23319/fup1.

Author: ueshin

python/pyspark/sql/tests.py

@@ -2860,7 +2860,7 @@ def test_create_dataframe_required_pandas_not_found(self):
         with QuietTest(self.sc):
             with self.assertRaisesRegexp(
                     ImportError,
-                    '(Pandas >= .* must be installed|No module named pandas)'):
+                    "(Pandas >= .* must be installed|No module named '?pandas'?)"):
                 import pandas as pd
                 from datetime import datetime
                 pdf = pd.DataFrame({"ts": [datetime(2017, 10, 31, 1, 1, 1)],