[r] Refactor current code base

[catriuspham]

This PR is highly about refactoring.

What have been done

• Separated integration tests into smaller parts
• Removed redundant tests (test_instance.py)
• Applied flake8
• Optimized imports
• Optimized code styles, indentations
• Use is in boolean and None comparison
• Noqa all F403 (from x import *)

[coveralls]

Coverage increased (+0.0009%) to 99.692% when pulling d5f5cff5e814bb5cf9d457da74bd76401d833d59 on chore/refactor into dc4cb483132277a5602594c5033d00629799403f on master.

[coveralls]

Coverage increased (+0.0009%) to 99.692% when pulling d4273d884a9be62e19a5ca0d68e247818d84a297 on chore/refactor into dc4cb483132277a5602594c5033d00629799403f on master.

[catriuspham]

@hieueastagile @NhanHo Please take a look

[NhanHo]

@catriuspham E712: foo is True or foo is False E711: foo is None

[hieueastagile]

I guess that we should fix it as:

x is True
x is False
x is None

For the import *, please check if it's needed or not, if not, just import all of them, if it's needed, just add a # NOQA to let flake8 ignore it.

[hieueastagile]

LGTM

[NhanHo]

LGTM

[coveralls]

Coverage increased (+0.0009%) to 99.692% when pulling e3f9df75e16296ce606c8cd9c23643982de4b362 on chore/refactor into dc4cb483132277a5602594c5033d00629799403f on master.

[coveralls]

Coverage increased (+0.0009%) to 99.701% when pulling e4c3d5310a0dca76d23009b4459be26cb741eabf on chore/refactor into f40aa943f4f6d17f74d114a5ffc51a884f9a51d4 on master.