also check dependencies for sklearn string

[amueller]

Fixes the remainder of #734.

We don't have the model at this point so we can't check the model.

[mfeurer]

Windows tests are failing (travis probably too, but that's still running):

_____________________ TestInit.test_get_extension_by_flow _____________________
[gw3] win32 -- Python 3.5.6 c:\miniconda35-x64\python.exe
self = <tests.test_extensions.test_functions.TestInit testMethod=test_get_extension_by_flow>
def test_get_extension_by_flow(self):
> self.assertIsNone(get_extension_by_flow(DummyFlow()))
tests\test_extensions\test_functions.py:68: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
openml\extensions\functions.py:51: in get_extension_by_flow
if extension_class.can_handle_flow(flow):
openml\extensions\sklearn\extension.py:75: in can_handle_flow
return cls._is_sklearn_flow(flow)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
cls = <class 'openml.extensions.sklearn.extension.SklearnExtension'>
flow = <tests.test_extensions.test_functions.DummyFlow object at 0x000000AAB3AC3A58>
@classmethod
def _is_sklearn_flow(cls, flow: OpenMLFlow) -> bool:
> if flow.dependencies is not None and "sklearn" in flow.dependencies:
E AttributeError: 'DummyFlow' object has no attribute 'dependencies'
openml\extensions\sklearn\extension.py:493: AttributeError
============================== warnings summary ===============================
c:\miniconda35-x64\lib\genericpath.py:19

[mfeurer]

Please fix the failing unit test.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@29a023c). Click here to learn what that means.
The diff coverage is 100%.

@@ Coverage Diff @@## develop #830 +/- ##
==========================================
Coverage ? 88.02% ==========================================
Files ? 36 Lines ? 4252 Branches ? 0 ==========================================
Hits ? 3743 Misses ? 509 Partials ? 0

Impacted Files	Coverage Δ
openml/extensions/sklearn/extension.py	91.3% <100%> (ø)

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