Deprecation warning due to invalid escape sequences

[tirkarthi]

Deprecation warning due to invalid escape sequences. Using raw strings or escaping them again helps in resolving this. Check https://github.com/asottile/pyupgrade/ for automatic fix of this.

find . -iname '*.py' | grep -Ev 'rdf4|tool|doc' | xargs -P4 -I{} python3.8 -Wall -m py_compile {}
./tests/xso/test_model.py:3073: DeprecationWarning: invalid escape sequence \.
  "<class '.+\.SubChild'> object is not a valid value"):
./tests/test_service.py:91: DeprecationWarning: invalid escape sequence \.
  "PATCHED_ORDER_AFTER must not be defined manually\. "
./tests/test_service.py:92: DeprecationWarning: invalid escape sequence \.
  "it is supplied automatically by the metaclass\."):
./tests/test_service.py:103: DeprecationWarning: invalid escape sequence \.
  "^_DEPGRAPH_NODE should not be defined manually\."):
./tests/avatar/test_service.py:237: DeprecationWarning: invalid escape sequence \.
  "^The image bytes can only be given for image/png data\.$"):
./tests/avatar/test_service.py:247: DeprecationWarning: invalid escape sequence \.
  "^Only one avatar image may be published directly\.$"):
./tests/avatar/test_service.py:257: DeprecationWarning: invalid escape sequence \.
  "^The given id does not match the SHA1 of the image data\.$"):
./tests/avatar/test_service.py:267: DeprecationWarning: invalid escape sequence \.
  "of the image data\.$"):
./aioxmpp/security_layer.py:22: DeprecationWarning: invalid escape sequence \ 
  """

[horazont]

@tirkarthi Thanks for bringing this up. Do you want to make a PR for this or shall I?