Regular expressions in gpsdatetime.py trigger syntaxwarnings in python 3.13

Upon import in python 3.13, the following warnings are emitted:

C:\Users\dteid\workspace\.venv\Lib\site-packages\gpsdatetime\gpsdatetime.py:36: SyntaxWarning: invalid escape sequence '\.'
  _re_iso = '^[12][0-9][0-9][0-9]-[01][0-9]-[0123][0-9]T[012][0-9]:[0-5][0-9]:[0-5][0-9](\.\d+)?Z$'
C:\Users\dteid\workspace\.venv\Lib\site-packages\gpsdatetime\gpsdatetime.py:37: SyntaxWarning: invalid escape sequence '\.'
  _re_rnx = '^[0-9][0-9] [ 01][0-9] [ 0123][0-9] [ 012][0-9] [ 0-5][0-9] [ 0-5][0-9](\.\d+)?$'
C:\Users\dteid\workspace\.venv\Lib\site-packages\gpsdatetime\gpsdatetime.py:38: SyntaxWarning: invalid escape sequence '\.'
  _re_rnx3 = '^[0-9][0-9][0-9][0-9] [ 01][0-9] [ 0123][0-9] [ 012][0-9] [ 0-5][0-9] [ 0-5][0-9](\.\d+)?$'

This is resolved by wrapping the strings as raw-strings, e.g. r'^[12]...'

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