[washingtonpost] Fix invalid escape sequence on Python 3.6

youtube_dl/extractor/washingtonpost.py

@@ -13,7 +13,7 @@ from ..utils import (
 class WashingtonPostIE(InfoExtractor):
     IE_NAME = 'washingtonpost'
     _VALID_URL = r'(?:washingtonpost:|https?://(?:www\.)?washingtonpost\.com/video/(?:[^/]+/)*)(?P<id>[\da-f]{8}-[\da-f]{4}-[\da-f]{4}-[\da-f]{4}-[\da-f]{12})'
-    _EMBED_URL = 'https?://(?:www\.)?washingtonpost\.com/video/c/embed/[\da-f]{8}-[\da-f]{4}-[\da-f]{4}-[\da-f]{4}-[\da-f]{12}'
+    _EMBED_URL = r'https?://(?:www\.)?washingtonpost\.com/video/c/embed/[\da-f]{8}-[\da-f]{4}-[\da-f]{4}-[\da-f]{4}-[\da-f]{12}'
     _TEST = {
         'url': 'https://www.washingtonpost.com/video/c/video/480ba4ee-1ec7-11e6-82c2-a7dcb313287d',
         'md5': '6f537e1334b714eb15f9563bd4b9cdfa',