[HOTFIX] Updating pytest dependency to fix error

[tallamjr]

It was discovered in builds from https://github.com/alan-turing-institute/dodo/pull/79
and https://github.com/alan-turing-institute/dodo/pull/78 a failure has occurred on Travis

Command:
$ pytest -rs --ignore=./tests/integration ./tests

Error:
TypeError: attrib() got an unexpected keyword argument 'convert'

It appears that pytest seems to have the package attrs as a dependency.

REF:
https://stackoverflow.com/questions/58189683/typeerror-attrib-got-an-unexpected-keyword-argument-convert

modified:   requirements.txt

[rkm]

If this fixes the CI build for the demo then go ahead and merge it in 👍

I'd like to take a closer look at this before merging back into dev/master later though, as there may be other breaking changes between pytest 4.1 -> 5.2. This should also be specified in the requirements-dev.txt file as well since its a development dependency only.

[tallamjr]

I'd like to take a closer look at this before merging back into dev/master later though, as there may be other breaking changes between pytest 4.1 -> 5.2.

Sounds good 👍 (For your reference this change was of pytest 4.1 -> 5.2 also took place in dodo a couple of weeks ago)

This should also be specified in the requirements-dev.txt file as well since its a development dependency only.

Just updated the commit, 698ba12 should address this

[tallamjr]

This PR corrects the initial failing tests outlined in issue #79, but this has revealed further failing tests.

Merging this PR such that a new ticket can be spawned to address these new problems