Deprecation warning due to invalid escape sequences in Python 3.8

Deprecation warnings are raised due to invalid escape sequences in Python 3.8 . Below is a log of the warnings raised during compiling all the python files. Using raw strings or escaping them will fix this issue.

I will add a PR

```
find . -iname '*.py' | xargs -P 4 -I{} python3.8 -Wall -m py_compile {}

./test/df_test.py:32: DeprecationWarning: invalid escape sequence \s
(filesystem, capacity, used, remaining, pct) = re.split("\s+", expected_output)
./snakebite/rpc_sasl.py:85: DeprecationWarning: invalid escape sequence \/
service = re.split('[\/@]', str(self.hdfs_namenode_principal))[0]
./snakebite/minicluster.py:204: DeprecationWarning: invalid escape sequence \d
m = re.match(".*Started MiniDFSCluster -- namenode on port (\d+).*", line)
./snakebite/minicluster.py:214: DeprecationWarning: invalid escape sequence \s
(perms, replication, owner, group, length, date, time, path) = re.split("\s+", line)
./snakebite/minicluster.py:236: DeprecationWarning: invalid escape sequence \s
fields = re.split("\s+", line)
./snakebite/minicluster.py:240: DeprecationWarning: invalid escape sequence \s
(length, path) = re.split("\s+", line)
./snakebite/minicluster.py:238: DeprecationWarning: invalid escape sequence \s
(length, space_consumed, path) = re.split("\s+", line)
./snakebite/minicluster.py:253: DeprecationWarning: invalid escape sequence \s
(_, dir_count, file_count, length, path) = re.split("\s+", line)
./snakebite/client.py:816: SyntaxWarning: "is not" with a literal. Did you mean "!="?
elif not load['error'] is '':

```