swagger_model.py: Fix invalid escape sequence in get_list_parameter_type().

swagger_model.py: Fix invalid escape sequence in get_list_parameter_type().

Recent python versions complain when backslashes in strings create invalid
escape sequences.  This causes issues for strings used as regex patterns like
`'^List\[(.*)\]$'` where you want the regex parser to treat `[` and `]`
as literals.  Double-backslashing is one way to fix it but simply converting
the string to a raw string `re.match(r'^List\[(.*)\]$', text)` is easier
and less error prone.