Fix 'invalid escape sequence' warning
Fix 'invalid escape sequence' warning

When running tests on py 3.8, I get the following warning:

  /home/amezin/dockerfile-parse/dockerfile_parse/parser.py:236: DeprecationWarning: invalid escape sequence \s
    line = re.sub('^\s*#\s*', '', line)

Apparently, this should be a raw literal

Signed-off-by: Aleksandr Mezin <76c1e6be84a1d2145428706eb61f38549c50daf3@gmail.com>

DBuildService/dockerfile-parse,DBuildService/dockerfile-parse