[Breaking] Rename data to X in predict_proba. (#6555)

New Scikit-Learn version uses keyword argument, and `X` is the predefined
keyword.

* Use pip to install latest Python graphviz on Windows CI.

tests/ci_build/conda_env/win64_test.yml

@@ -9,7 +9,6 @@ dependencies:
 - scikit-learn
 - pandas
 - pytest
-- python-graphviz
 - boto3
 - hypothesis
 - jsonschema
@@ -17,3 +16,4 @@ dependencies:
 - pip:
   - cupy-cuda101
   - modin[all]
+  - graphviz