sizeof test: increase tolerance (#825)

Closes #824 by increasing the tolerance of `sizeof` test of `cudf.DataFrame`.

In the test we clear the `ProxyObject` cache, which makes `sizeof` measure the serialized dataframe. I order to avoid deserializing on every `sizeof` call, we accept this discrepancy between  `sizeof` serialized and deserialized objects.

Authors:
  - Mads R. B. Kristensen (https://github.com/madsbk)

Approvers:
  - Peter Andreas Entschev (https://github.com/pentschev)

URL: #825

dask_cuda/tests/test_proxy.py

@@ -605,6 +605,8 @@ def test_sizeof_cudf():
     pxy._pxy_serialize(serializers=("dask",))
     assert a_size == pytest.approx(sizeof(pxy))
     assert pxy._pxy_get().is_serialized()
+    # By clearing the cache, `sizeof(pxy)` now measure the serialized data
+    # thus we have to increase the tolerance.
     pxy._pxy_cache = {}
-    assert a_size == pytest.approx(sizeof(pxy))
+    assert a_size == pytest.approx(sizeof(pxy), rel=1e-2)
     assert pxy._pxy_get().is_serialized()