Fix invalid escape sequence warnings

[Mirochill]

Summary

• convert the invalid escape sequence regex literals reported in Importing ete3 generates many SyntaxWarnings with python 3.12 #771 to raw strings
• make affected documentation strings raw where their ASCII tree examples intentionally contain backslashes
• keep control-character literals such as tab/newline/carriage return intact where they are part of parser character sets

Fixes #771.

Validation

• git diff --check
• Not run locally