Unpin matplotlib

[bjlittle]

The matplotlib package has been pinned to <v3.3.0 to temporarily avoid test failures within iris due to the latest release v3.3.0.

See #3758.

[trexfeathers]

Should hopefully be addressed by the unpin proj task - see comment on #3154.

[tkknight]

The read the docs yml conf file also will need to be unpinned.

[trexfeathers]

Closed by #3811