ArteTV support broken by unicode encoded JSON URLs

[desseim]

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2019.07.02
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.arte.tv/fr/videos/065424-071-A/blow-up-c-est-quoi-claudia-cardinale/']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2019.07.02
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.18362
[debug] exe versions: ffmpeg N-90169-gf4709f1b7b, ffprobe N-90169-gf4709f1b7b
[debug] Proxy map: {}
[arte.tv:+7] blow-up-c-est-quoi-claudia-cardinale: Downloading webpage
[download] Downloading playlist: None
[arte.tv:+7] playlist None: Collected 0 video ids (downloading 0 of them)
[download] Finished downloading playlist: None

Description

Since yesterday 2019/07/02, URL of videos on the arte.tv website, which were working until then, stopped getting downloaded.
I don't know what changed, but I figured that the extractor, in _extract_from_webpage(), looks for a JSON URL with find_iframe_url() by searching for a pattern starting with <iframe but matches nothing as JSON data are unicode-escaped in the response stored in the webpage variable, and would thus appear as \\u003Ciframe instead.
I found that by unescaping the retrieved webpage as follows, expected eventual behavior (i.e. videos getting downloaded) is restored:

--- arte.old.py
+++ arte.new.py
@@ -215,1 +215,1 @@
-        webpage = self._download_webpage(url, video_id)
+        webpage = self._download_webpage(url, video_id, encoding="unicode_escape")

I don't know whether this is the piece of the extractor that got broken by a recent change in the website response, but I can confirm it restores video downloading functionality for the URLs that recently stopped working.
Please note that most _TESTS URLs are now outdated and it's better to test with fresh URLs from https://www.arte.tv.

[desseim]

Duplicates #21614 I guess.
Sorry I didn't catch it when reporting.