Deprecation warning due to invalid escape sequences

[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 -Ev 'test.py' | xargs -P4 -I{} python3.8 -Wall -m py_compile {}
./hcluster/distance.py:1648: DeprecationWarning: invalid escape sequence \c
  '(k \choose 2)=n)!') % name)
./hcluster/distance.py:1652: DeprecationWarning: invalid escape sequence \c
  'be a k such that (k \choose 2)=n)!')