PY-21697 Handle sequences like '\\''' in multiline strings in Python lexer

Previously, they would get matched as '\ followed by \', and then,
two remaining quotes. Therefore the literal was considered unterminated
by the lexer. It, in turn, broke multiple assertions in
PyStringLiteralLexer because it used to find different string's end than
PythonHighlightingLexer and rendered the whole editor unresponsive.

python/testData/editing/tripleQuotesInsideTripleQuotedStringLiteral.after.py

@@ -0,0 +1,3 @@
+s = '''
+'\\''''
+'''

python/testData/editing/tripleQuotesInsideTripleQuotedStringLiteral.py

@@ -0,0 +1,3 @@
+s = '''
+'\\<caret>''
+'''