cassandra/cluster.py: use raw string when appropriate

Python complains at seeing
```py
re.compile(r'^\s*BEGIN\s+[a-zA-Z]*\s*BATCH', re.UNICODE)
```
```
<>:1: SyntaxWarning: invalid escape sequence '\s'
```

but the interpreter continues on, and take "\s" as it
is without escaping it. still, it's not a valid string
literal.

because "\s" is not an escape sequence, while "\\s" is,
but we don't have to escape "\" here, we can just use the
raw string. simpler this way.

in this change, we trade the invalid escape sequence with
a raw string to silence this warning.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Author: tchaikov

cassandra/cluster.py

@@ -5451,7 +5451,7 @@ def cancel_continuous_paging(self):
         except AttributeError:
             raise DriverException("Attempted to cancel paging with no active session. This is only for requests with ContinuousdPagingOptions.")
 
-    batch_regex = re.compile('^\s*BEGIN\s+[a-zA-Z]*\s*BATCH')
+    batch_regex = re.compile(r'^\s*BEGIN\s+[a-zA-Z]*\s*BATCH')
 
     @property
     def was_applied(self):