Unpin upper bound for iris (previously was at <3.0.4)

[valeriupredoi]

Description

Unpinning iris and let it freeeedooom 🏴󠁧󠁢󠁳󠁣󠁴󠁿 - ie remove the upper bound at 3.0.4. This will only work proper when we have Core specs with the upper bound removed too. I'll do a Core PR with the same shtuff asap.

• Closes ESMValCore: pin on iris<3.0.4 to still allow (formal) support for Python=3.6 #2245
• Link to documentation:

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[bouweandela]

Conda build test ran out of memory, but everything else looks fine.