we use "\." to escape "." in a regular expression. but "\" should
be escaped as well, CPython accepts "\." after trying to find
an escaped character of "\." but failed, and leave "\." as it is.
but it complains:
```
/home/kefu/dev/scylladb/scripts/coverage.py:107: SyntaxWarning: invalid escape sequence '\.'
input_file_re_str = f"(.+)\.profraw(\.{__DISTINCT_ID_RE})?"
```
in this change, we use raw string to avoid escaping "\" in
the regular expression.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
c3c715236d