Make W605 point to the invalid sequence

Instead of having W605 point to the beginning of the string literal,
make it point to the precise line and column of the invalid escape sequence.

This is more helpful when you have multiline string literals.

Author: mgedmin

pycodestyle.py

@@ -1522,7 +1522,7 @@ def python_3000_invalid_escape_sequence(logical_line, tokens):
 
     for token_type, text, start, end, line in tokens:
         if token_type == tokenize.STRING:
-            orig_start = start
+            start_line, start_col = start
             quote = text[-3:] if text[-3:] in ('"""', "'''") else text[-1]
             # Extract string modifiers (e.g. u or r)
             quote_pos = text.index(quote)
@@ -1535,8 +1535,13 @@ def python_3000_invalid_escape_sequence(logical_line, tokens):
                 while pos >= 0:
                     pos += 1
                     if string[pos] not in valid:
+                        line = start_line + string[:pos].count('\n')
+                        if line == start_line:
+                            col = start_col + len(prefix) + len(quote) + pos
+                        else:
+                            col = pos - string.rfind('\n', 0, pos) - 1
                         yield (
-                            orig_start,
+                            (line, col - 1),
                             "W605 invalid escape sequence '\\%s'" %
                             string[pos],
                         )

testsuite/W60.py

@@ -13,16 +13,23 @@
     x = 0
 #: W604
 val = `1 + 2`
-#: W605:1:9
+#: W605:1:10
 regex = '\.png$'
-#: W605:1:9
+#: W605:2:1
 regex = '''
 \.png$
 '''
-#: W605:2:5
+#: W605:2:6
 f(
     '\_'
 )
+#: W605:4:6
+"""
+multi-line
+literal
+with \_ somewhere
+in the middle
+"""
 #: Okay
 regex = r'\.png$'
 regex = '\\.png$'