Fix SyntaxWarning: invalid escape sequence '\B' (#674)

Use raw triple double quotes, because backslashes are being
used in the docstring.

This was the warning:
(...)/site-packages/ahbicht/expressions/ahb_expression_parser.py:29:
SyntaxWarning: invalid escape sequence '\B'
CONDITION_EXPRESSION: /(?!\BU\B)[\[\]\(\)U∧O∨X⊻\d\sP\.UBn]+/i

Author: ckrybus

src/ahbicht/expressions/ahb_expression_parser.py

@@ -15,7 +15,7 @@
 from ahbicht.expressions.sanitizer import sanitize_expression
 from ahbicht.utility_functions import tree_copy
 
-GRAMMAR = """
+GRAMMAR = r"""
 ahb_expression: modal_mark_expression+
                 | prefix_operator_expression
                 | requirement_indicator