[BUG] Deprecation warning due to invalid escape sequences

[tirkarthi]

Describe the bug

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/

To Reproduce

./tests/test_markup.py:26: DeprecationWarning: invalid escape sequence \[
  assert escape("foo[bar]") == "foo\[bar]"
./tests/test_markup.py:30: DeprecationWarning: invalid escape sequence \[
  result = list(_parse("[foo]hello[/foo][bar]world[/]\[escaped]"))
./rich/markup.py:50: DeprecationWarning: invalid escape sequence \[
  return markup.replace("[", "\[")

Platform
What platform (Win/Linux/Mac) are you running on? What terminal software are you using. Which version of Rich?