Include a timeout for session.post in sync_execute

[mmangus]

In sync_execute, commands are executed by making requests to the Upstash REST API like this...

response = session.post(url, headers=headers, json=command).json()

...where session is a requests.Session instance.

As indicated by the requests documentation, this call should include a timeout to avoid blocking for long periods in the event of networking trouble:

Nearly all production code should use this parameter [timeout] in nearly all requests. Failure to do so can cause your program to hang indefinitely

For the Redis workloads I typically have, no response from Redis is better than a slow response from Redis. The client should at least allow users to configure a timeout for these requests, and in my opinion should also have a default timeout enabled without user intervention.