Update Julia version Compat

[PaulDebus]

Increase lowest supported Julia version to 1.6
Set Compatibility to ^1.6 so anything <2 works
Update the CI to use Julia 1.6 instead of 1.4

Should this trigger a new release of this package? I would wait until we sort out #37 and #38

[nlw0]

It's a good question about the version, according to semver.org we should mostly focus on whether there were changes to the API... I propose we call this a bug and just upgrade the patch version.

I noticed tests seem to be running on windows but not linux. Maybe just a different version of PyPlot can solve it? I can't tell which specified versions of the other packages are relevant as well... Perhaps we could give it a try to upgrade all of them?

[PaulDebus]

Yeah, increasing the patch version sounds fine to me.
Regarding the test failing on ubuntu: I looked into it and it looks like the error is also occuring in PyPlot
JuliaPy/PyPlot.jl#571
I see two options: wait for them to figure out the issue and the solution should propagate to us or we cut the old ties, set the minimum version to 1.9, move PyPlot into a weak dependency and call it a day. What do you think?

[PaulDebus]

A general look at the dependencies and updating the incompatibilities is also a good idea, although not directly related here IMHO. This should be a separate cleanup step