Fix DeprecationWarning: invalid escape sequence

[BoboTiG]

Fixes several DeprecationWarning:

prettytable.py:76: DeprecationWarning: invalid escape sequence \[
  _re = re.compile("\033\[[0-9;]*m")
prettytable.py:929: DeprecationWarning: invalid escape sequence \{
  self.vertical_char = random.choice("~!@#$%^&*()_+|-=\{}[];':\",./;<>?")
prettytable.py:930: DeprecationWarning: invalid escape sequence \{
  self.horizontal_char = random.choice("~!@#$%^&*()_+|-=\{}[];':\",./;<>?")
prettytable.py:931: DeprecationWarning: invalid escape sequence \{
  self.junction_char = random.choice("~!@#$%^&*()_+|-=\{}[];':\",./;<>?")