Fix GithubException.__str__ decoding bytes data (#3494)

`GithubException.__str__` calls `json.dumps(self.data)` directly, which
raises `TypeError: Object of type bytes is not JSON serializable` when
the exception is constructed from a non-JSON response body that
`requests` returns as `bytes` (e.g. an HTML rate-limit page on a 403).
Decode bytes to text before serialising so callers can log or stringify
these exceptions without crashing.

Fixes #3492.

Signed-off-by: Sai Asish Y <say.apm35@gmail.com>