test-requirements.txt: Use pytest 3.6
test-requirements.txt: Use pytest 3.6

Also upgrades pytest-timeout as 1.2 was incompatible
with pytest 3.6.
pytest-timeout 1.3 is incompatible with pytest 3.5 and lower.

Related to https://github.com/coala/coala/issues/5525
Closes https://github.com/coala/coala/issues/5543

SanketDG/coala,coala/coala,coala/coala,coala-analyzer/coala,jayvdb/coala,coala/coala,coala-analyzer/coala,SanketDG/coala,coala-analyzer/coala,SanketDG/coala,jayvdb/coala,jayvdb/coala