Deprecation warning due to invalid escape sequences in Python 3.7

[tirkarthi]

Deprecation warnings are raised due to invalid escape sequences. This can be fixed by using raw strings or escaping the literals. pyupgrade also helps in automatic conversion : https://github.com/asottile/pyupgrade/

find . -iname '*.py' | grep -v example | xargs -P 4 -I{} python3.8 -Wall -m py_compile {}
./gtts/tokenizer/core.py:6: DeprecationWarning: invalid escape sequence \.
  """Builds regex using arguments passed into a pattern template.
./gtts/tokenizer/core.py:216: DeprecationWarning: invalid escape sequence \,
  """An extensible but simple generic rule-based tokenizer.