Remove version pin on proj4.

[pp-mo]

DO NOT MERGE (yet)

Check to see if tests still fail when removing this constraint

[pp-mo]

if tests still fail when removing this constraint
No they don't !!

So I do think this is obsolete.
This is stopping (making skip) all the tests of the iris-grib interface.
Fixing those will unblock #2608

That still requires additional work, but I think this part is crucial.
Coming up ... use eccodes + get later iris-grib, ( ? with conda in place of pip ?).

[bjlittle]

@pp-mo We're using pip and at the moment eccodes-python is only available on PyPI - this is the only version that works in Python3 at the moment. Offline we can help the ECMWF package with conda.

[pp-mo]

We're using pip and at the moment eccodes-python is only available on PyPI

Ok, I was hoping to simplify here, but that explains it.
I can probably modify this, copying the iris-grib testing approach, to get Python3 testing working also.

In the meantime, I see we now have a bunch of things that look like they aren't compatible between gribapi and eccodes -- but presumably none of these are tested in iris-grib itself.
Some of this testing probably still wants to be migrated out of iris : We still have a heap of tech debt in iris-grib!!

[bjlittle]

@pp-mo So what are you intentions for this particular PR?

Seems like you are pulling on quite a few strings that reach across into iris-grib, eccodes,python-eccodes (Python2) and (confusingly) eccodes-python (Python3).

[pp-mo]

Let's kill this off
and replace with a PR that just unpins proj4 like it says on the box 😉 == #3374
and create a separate issue to address getting iris-grib interface testing back up == #3375