chore: update regexp to treat string as raw, eliminating warnings

[ndobbs]

Fixes: #57

This PR attempts to fix a SyntaxWarning that's raised from the library due to an invalid escape sequence in the pattern used in re.findall()

The backslash before the square bracket in the string was not being handled properly. This is resolved by prefixing it with r, suring that the backslash is treated literally.

error output

/venv/lib/python3.12/site-packages/metabase_api/metabase_api.py:211: SyntaxWarning: invalid escape sequence '\['
  res = re.findall("\['field', .*?\]", query_data_str)