Fix invalid escape sequence in regex
Hello.
Lint tools told me about `\` in regex. I think it's better to be raw strings.

pycodestyle says me:
```
playhouse/cockroachdb.py:71:48: W605 invalid escape sequence '\d'
playhouse/cockroachdb.py:71:52: W605 invalid escape sequence '\.'
playhouse/cockroachdb.py:71:55: W605 invalid escape sequence '\d'
playhouse/cockroachdb.py:71:59: W605 invalid escape sequence '\.'
playhouse/cockroachdb.py:71:62: W605 invalid escape sequence '\d'
```

Also pylint says:
```
playhouse/cockroachdb.py:71:17: W1401: Anomalous backslash in string: '\d'. String constant might be missing an r prefix. (anomalous-backslash-in-string)
playhouse/cockroachdb.py:71:21: W1401: Anomalous backslash in string: '\.'. String constant might be missing an r prefix. (anomalous-backslash-in-string)
playhouse/cockroachdb.py:71:24: W1401: Anomalous backslash in string: '\d'. String constant might be missing an r prefix. (anomalous-backslash-in-string)
playhouse/cockroachdb.py:71:28: W1401: Anomalous backslash in string: '\.'. String constant might be missing an r prefix. (anomalous-backslash-in-string)
playhouse/cockroachdb.py:71:31: W1401: Anomalous backslash in string: '\d'. String constant might be missing an r prefix. (anomalous-backslash-in-string)
```

Reference https://stackoverflow.com/questions/19030952/pep8-warning-on-regex-string-in-python-eclipse