Depending on if a "stream" or the "encoding" is given, the functions
either return None, str/unicode or bytes.
Use @overload fix distinguish those cases.
Also fix the functions using **kwds as they delegate their work to the
more generic functions: copy their signatures.
* yaml.dump(): Drop distinguishing encoding
As far as I know it's currently impossible to use "overloads with return
types depending on optional arguments" (Issue #5621). As "encoding" is
in the middle of 11 optional arguments, it would require ~ 2^6 overloads
for each of those 6 functions.
For now just return Any and wait for Issue #5621 to get fixed.
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