Set session scope for 'one_compressed_file' fixture

Set session scope for the 'one_compressed_file' fixture to fix
hypothesis health check:

```
E                               hypothesis.errors.FailedHealthCheck: test/test_simple_compression.py::test_streaming_compression_flush uses the 'one_compressed_file' fixture, which is reset between function calls but not between test cases generated by `@given(...)`.  You can change it to a module- or session-scoped fixture if it is safe to reuse; if not we recommend using a context manager inside your test function.  See https://docs.pytest.org/en/latest/fixture.html#sharing-test-data for details on fixture scope.
E                               See https://hypothesis.readthedocs.io/en/latest/healthchecks.html for more information about this. If you want to disable just this health check, add HealthCheck.function_scoped_fixture to the suppress_health_check settings for this test.
```

Author: mgorny

test/conftest.py

@@ -25,6 +25,6 @@ def simple_compressed_file(request):
     return (file[:-len('.compressed')], file)
 
 
-@pytest.fixture()
+@pytest.fixture(scope='session')
 def one_compressed_file():
     return os.path.join(TEST_DATA_DIR, 'alice29.txt')