Support Black 20

Use the option --target-version insteead --py36, which was removed
in Black 20.8b0.

The option --target-version was added in Black 19.3b0, so blacken now
requires at least this version of Black.

See psf/black#1236

blacken.el

@@ -112,7 +112,7 @@ Return black process the exit code."
                                ('fill fill-column)
                                (t blacken-line-length)))))
    (when blacken-allow-py36
-     (list "--py36"))
+     (list "--target-version" "py36"))
    (when blacken-fast-unsafe
      (list "--fast"))
    (when blacken-skip-string-normalization