Add `timeout` to `Problems.get_problem()`; enable long polling

[randomir]

I wasn't aware timeout is supported on get problem endpoint at the time I worked on #649.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.87%. Comparing base (f2a1d07) to head (6bc23b7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #657 +/- ## ======================================= Coverage 88.86% 88.87% ======================================= Files 42 42 Lines 4796 4799 +3 ======================================= + Hits 4262 4265 +3 Misses 534 534 ```

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