CI should work on python 3.6

[pyalex]

Signed-off-by: Oleksii Moskalenko moskalenko.alexey@gmail.com

What this PR does / why we need it:

To guarantee compatibility with python 3.6 (as declared in our setup.py) this PR downgrades python version to 3.6 in CI docker image.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[khorshuheng]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: khorshuheng, pyalex

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[khorshuheng]

/test test-end-to-end-sparkop

[feast-ci-bot]

New changes are detected. LGTM label has been removed.

[feast-ci-bot]

@pyalex: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
test-end-to-end-sparkop	48fdb08	link	/test test-end-to-end-sparkop

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