Merge pull request #28195 from harsh-suman5:fix-coverage-regex

Fix regex escape warning in _coverage.py by using raw string #2819

while i am running '_coverage.py' I noticed there is a syntax warning:"\." It is an invalid escape sequence
The regex was written as a normal string with '\.' which is not valid escape in string literals and give problems and error in a new python versions. 
To fix this switch to raw string(r'cv2?\.\w+') which can passes the backslashes directly to the regex engine. This removes the warning and keeps the code  and pattern working properly.

This is a small fix but important for future compatibilities.

samples/python/_coverage.py

@@ -18,7 +18,7 @@ if __name__ == '__main__':
     for fn in glob('*.py'):
         print(' --- ', fn)
         code = open(fn).read()
-        found |= set(re.findall('cv2?\.\w+', code))
+        found |= set(re.findall(r'cv2?\.\w+', code))
 
     cv2_used = found & cv2_callable
     cv2_unused = cv2_callable - cv2_used