Fix escape sequences in Python's strings (rouge-ruby#1508)

Version 3.18.0 of Rouge included a new mechanism for handling strings
in the Python lexer. One of the consequences of that change was that
raw strings would break if they included 'invalid' escape sequences.
This is a mistake as raw strings do not have 'invalid' escape sequences.

This commit fixes this error by changing the approach that the Python
lexer takes to escape sequences more generally. Rather than dividing
recognised and unrecognised escape sequences into 'valid' and 'invalid',
it simply treats unrecognised escape sequences as ordinary strings.

The result of this is that all escape sequences in raw strings produce
`Str` tokens. In other types of strings, recognised escape sequences
produce `Str::Escape` tokens and unrecognised escape sequences produce
`Str` tokens.

lib/rouge/lexers/python.rb

@@ -186,14 +186,7 @@ def current_string
           end
         end
 
-        rule %r/\\/ do |m|
-          if current_string.type? "r"
-            token Str
-          else
-            token Str::Interpol
-          end
-          push :generic_escape
-        end
+        rule %r/(?=\\)/, Str, :generic_escape
 
         rule %r/{/ do |m|
           if current_string.type? "f"
@@ -206,23 +199,22 @@ def current_string
       end
 
       state :generic_escape do
-        rule %r(
+        rule %r(\\
           ( [\\abfnrtv"']
           | \n
+          | newline
           | N{[a-zA-Z][a-zA-Z ]+[a-zA-Z]}
           | u[a-fA-F0-9]{4}
           | U[a-fA-F0-9]{8}
           | x[a-fA-F0-9]{2}
           | [0-7]{1,3}
           )
         )x do
-          if current_string.type? "r"
-            token Str
-          else
-            token Str::Escape
-          end
+          token (current_string.type?("r") ? Str : Str::Escape)
           pop!
         end
+
+        rule %r/\\./, Str, :pop!
       end
 
       state :generic_interpol do

spec/visual/samples/python

@@ -47,6 +47,7 @@ def baz():
     '\UaaaaAF09'
     '\xaf\xAF\x09'
     '\007'
+    '.*\[p00t_(d\d{4})\].*' # There are no escape sequences in this string
 
 # escaped characters in raw strings
 def baz():
@@ -56,6 +57,7 @@ def baz():
     r'\UaaaaAF09'
     r'\xaf\xAF\x09'
     r'\007'
+    r'.*\[p00t_(d\d{4})\].*'
 
 # line continuations
 apple.filter(x, y)