PY-31442 Better handling of single backslashes (e.g. before braces) in literal parts

There is actually no need to match them together with braces and then return
the second character back to the stream. The longest match rule will ensure
that all allowed escape sequences will be recognised as one unit, so it seems
safe to match backslash individually for all remaining cases.

python/testData/psi/UnterminatedFStringWithTrailingBackslashInFormatPart.txt

@@ -5,7 +5,7 @@ PyFile:UnterminatedFStringWithTrailingBackslashInFormatPart.py
     PsiWhiteSpace(' ')
     PsiElement(Py:EQ)('=')
     PsiWhiteSpace(' ')
-    PyStringLiteralExpression: {42:
+    PyStringLiteralExpression: {42:\
       PyFormattedStringNode
         PsiElement(Py:FSTRING_START)('f'')
         PyFStringFragment
@@ -14,8 +14,8 @@ PyFile:UnterminatedFStringWithTrailingBackslashInFormatPart.py
             PsiElement(Py:INTEGER_LITERAL)('42')
           PyFStringFragmentFormatPart
             PsiElement(Py:FSTRING_FRAGMENT_FORMAT_START)(':')
+            PsiElement(Py:FSTRING_TEXT)('\')
           PsiErrorElement:} expected
             <empty list>
-  PsiElement(BAD_CHARACTER)('\')
-  PsiErrorElement:Statement expected, found BAD_CHARACTER
-    <empty list>
+        PsiErrorElement:' expected
+          <empty list>

python/testData/psi/UnterminatedFStringWithTrailingBackslashInLiteralPart.txt

@@ -5,11 +5,9 @@ PyFile:UnterminatedFStringWithTrailingBackslashInLiteralPart.py
     PsiWhiteSpace(' ')
     PsiElement(Py:EQ)('=')
     PsiWhiteSpace(' ')
-    PyStringLiteralExpression: 
+    PyStringLiteralExpression: \
       PyFormattedStringNode
         PsiElement(Py:FSTRING_START)('f'')
-        PsiErrorElement:unexpected f-string token
-          <empty list>
-  PsiElement(BAD_CHARACTER)('\')
-  PsiErrorElement:Statement expected, found BAD_CHARACTER
-    <empty list>
+        PsiElement(Py:FSTRING_TEXT)('\')
+        PsiErrorElement:' expected
+          <empty list>