Don't pin Proj4

[pp-mo]

Remove the pin "proj4<5", introduced in #3150

Background:
#3150 fixed problems emerging with cartopy and Proj4 v 5.1
Apparently triggered by new builds on conda_forge (though also anaconda?).
A new build of cartopy 0.16, though no new version.

Needs fixing properly, but probably better done in cartopy ?
Possibly, cartopy may not work properly with Proj4 >= 5.
Which said, the newest build of cartopy 0.16 may be effectively invalid

• as here , lists as "py27h81b52dc_1 conda-forge",

[pp-mo]

UPDATE:
despite relaxing the pin, we still have problem with this, since proj4 v6 came out + causes a variety of problems.

So for the future : we still need to remove the pin
( but now it means fixing proj4 v6 compatibility )

[trexfeathers]

This is now a higher priority issue for fixing

Why

The proj4<6 pin now prevents Conda installing Iris with Cartopy>=0.18 - compare the installed packages for the latest Travis build against this experimental unpinned build.

Cartopy<0.18 also requires Matplotlib<3.3 - see #3759. So pins begetting more pins, each of which brings extra risk of future tech debt inflation (like what has already happened here).

It is also no longer possible for a pin of this sort to work - Iris' dependencies* are now referencing the proj package - this having replaced the proj4 package - and proj cannot be pinned back to the desired point since it 'took over' where proj4 stopped.

How

• Thankfully the previously mentioned problems with Cartopy's tests have since been fixed 🎉🏆
• Fix TestMappingSubRegion test - proj4>=6 broke this 2 years ago and proj still breaks it now.
  Might just be that the plot size has changed?
• Investigate other test failures seen in the recent experimental unpinned build - may or may not be related to PROJ.
  Could possibly also be Matplotlib>=3.3?

---

* The current dependency tree has proj being brought in via the gdal recipe (cartopy recently changed to include gdal in its recipe's host requirements, so no longer has any recipe references to proj).

[trexfeathers]

Closed by #3762