[deviantart] pin API version (#1611)

'/gallery/folders' in the newest version doesn't include subfolders.

It probably only needs the right query parameter to do so, but that
doesn't seem to be documented anywhere.

gallery_dl/extractor/deviantart.py

@@ -918,7 +918,7 @@ class DeviantartOAuthAPI():
     def __init__(self, extractor):
         self.extractor = extractor
         self.log = extractor.log
-        self.headers = {}
+        self.headers = {"dA-minor-version": "20200519"}
 
         self.delay = extractor.config("wait-min", 0)
         self.delay_min = max(2, self.delay)