DEP Remove file_id deprecation warning

[stephen-hoover]

There was no way for users to avoid this deprecation warning when calling civis.io.civis_file_to_table, and it was also triggered by other functions which called civis_file_to_table (such as dataframe_to_civis). The original thought had been to replace the singular file_id parameter with a plural file_ids parameter in v2, but per discussion in #360, the singular will be acceptable.

Closes #360.

[stephen-hoover]

@mheilman , flake8 v3.8 came out yesterday, and there's new checks which civis-python fails:

civis/run_joblib_func.py:108:19: F523 '...'.format(...) has unused arguments at position(s): 1
civis/ml/_model.py:199:30: E741 ambiguous variable name 'l'
civis/ml/_model.py:205:48: E741 ambiguous variable name 'l'
civis/ml/tests/test_model.py:377:43: E741 ambiguous variable name 'l'
civis/tests/test_io.py:1094:25: E741 ambiguous variable name 'l'
civis/tests/test_io.py:1115:34: E741 ambiguous variable name 'l'

What's your preferred resolution (cap flake8 version, fix flake8 errors, ignore flake8 errors), and should it be in this PR or another? Those things look relatively easy to fix, and at least the unused format parameter should be fixed.

[mheilman]

I made a separate PR to deal with the flake8 stuff: #394

(It seemed best to make things work with flake8 3.8 since one of the things it flagged was a minor bug in "logging" in the joblib runner)

[mheilman]

LGTM