include 'zstd' in Accept-Encoding header when supported

… and slightly update optional dependency list

gallery_dl/extractor/common.py

@@ -390,6 +390,8 @@ class Extractor():
             headers["Accept-Encoding"] = "gzip, deflate, br"
         else:
             headers["Accept-Encoding"] = "gzip, deflate"
+        if ZSTD:
+            headers["Accept-Encoding"] += ", zstd"
 
         referer = self.config("referer", self.referer)
         if referer:
@@ -990,6 +992,12 @@ try:
 except AttributeError:
     BROTLI = False
 
+# detect zstandard support
+try:
+    ZSTD = urllib3.response.HAS_ZSTD
+except AttributeError:
+    ZSTD = False
+
 # set (urllib3) warnings filter
 action = config.get((), "warnings", "default")
 if action: