Fix596 dependencies

[PGijsbers]

Remove oslo.concurrency, nbformat and pytest from general package dependencies, as they are only used for testing (and are this under the extra_requires 'test' flag).
I also updated the documentation (as I was looking for when any python setup.py install was mentioned). Have to hold out to see if it still works in CI setting.

Locally runs as expected (downloads task, data, flow, run):

import openml
from sklearn.tree import DecisionTreeClassifier
t = openml.tasks.get_task(59)
r = openml.runs.run_model_on_task(DecisionTreeClassifier(), t)
run = openml.runs.get_run(10160808)

This moves the package dependencies (without test environment) from:
Successfully installed Babel-2.6.0 PyYAML-5.1 atomicwrites-1.3.0 attrs-19.1.0 certifi-2019.3.9 chardet-3.0.4 debtcollector-1.21.0 decorator-4.4.0 fasteners-0.14.1 idna-2.8 ipython-genutils-0.2.0 iso8601-0.1.12 jsonschema-3.0.1 jupyter-core-4.4.0 liac-arff-2.4.0 monotonic-1.5 more-itertools-7.0.0 nbformat-4.4.0 netaddr-0.7.19 netifaces-0.10.9 numpy-1.16.2 openml oslo.concurrency-3.29.1 oslo.config-6.8.1 oslo.i18n-3.23.1 oslo.utils-3.40.3 pandas-0.24.2 pathlib2-2.3.3 pbr-5.1.3 pluggy-0.9.0 py-1.8.0 pyparsing-2.4.0 pyrsistent-0.14.11 pytest-4.4.1 python-dateutil-2.8.0 pytz-2019.1 requests-2.21.0 rfc3986-1.2.0 scikit-learn-0.20.3 scipy-1.2.1 six-1.12.0 stevedore-1.30.1 traitlets-4.3.2 urllib3-1.24.1 wrapt-1.11.1 xmltodict-0.12.0

to
Successfully installed certifi-2019.3.9 chardet-3.0.4 idna-2.8 liac-arff-2.4.0 numpy-1.16.2 openml pandas-0.24.2 python-dateutil-2.8.0 pytz-2019.1 requests-2.21.0 scikit-learn-0.20.3 scipy-1.2.1 six-1.12.0 urllib3-1.24.1 xmltodict-0.12.0

:)

[codecov-io]

Codecov Report

Merging #675 into develop will increase coverage by 0.01%.
The diff coverage is 90.9%.

@@             Coverage Diff             @@
##           develop     #675      +/-   ##
===========================================
+ Coverage    90.85%   90.87%   +0.01%     
===========================================
  Files           36       36              
  Lines         3533     3749     +216     
===========================================
+ Hits          3210     3407     +197     
- Misses         323      342      +19

Impacted Files	Coverage Δ
openml/runs/functions.py	83.94% <100%> (+0.33%)	⬆️
openml/tasks/functions.py	85.16% <100%> (-0.38%)	⬇️
openml/flows/functions.py	88.09% <100%> (-0.1%)	⬇️
openml/utils.py	91.27% <82.6%> (-1.59%)	⬇️
openml/datasets/functions.py	95.37% <93.75%> (-0.07%)	⬇️
openml/testing.py	95.61% <0%> (+0.28%)	⬆️
openml/extensions/sklearn/extension.py	90.28% <0%> (+0.44%)	⬆️

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