Make sure to not call sync IO functions inside async functions

[Krismix1]

I found that several sync IO functions are used inside of async functions. This is not good, as sync IO may block the eventloop for undetermined amount of time.
I had to add a new _a_token_info that mimics _token_info; the only difference is that it calls the async alternatives inside.
I think those should be all the places where a sync IO was made.

I also found another discrepancy between requests and httpx - httpx does not handle sets for form-data requests.
Encoding in requests vs httpx. httpx doesn't mention anything about support or lack of it for sets. Perhaps it could be reasonable for it to be fixed upstream.
Luckily some tests for a_uma_permissions caught this.

For now I fixed the problem inside the function itself.
The only other place I see where this could need a similar fix is KeycloakOpenID.a_token, which takes a user-supplied dict (the extra) and passes it as is to httpx.

[ryshoooo]

Thanks a lot @Krismix1 LGTM