[diagnotics_analysis] fix type comparison for python3

[mikaelarguedas]

In python3, self._stats[name]['fields'].keys() returns a dict_keys object, so the comparison to a list is never True.

PS: the default branch of this depo is indigo-devel making it the default target for PRs, should it be noetic-devel instead ?

[g-gemignani]

Thanks for the PR.
About the default branch, unfortunately I do not have the rights to do so. Maybe @trainman419 can do that?

[trainman419]

Ok; I've updated the default branch to noetic-devel

[mikaelarguedas]

Awesome, thank you both for the prompt reply 👍

[g-gemignani]

Thank you @trainman419