MNT: remove _hold shims to support basemap + cartopy

[tacaswell]

The putter was added in 3.0.1 to fix all released versions of cartopy.
The setter was added in 3.0.2 to fix basemap's handling of the API
change.

All are being removed for 3.1

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[WeatherGod]

Has cartopy fixed this issue in their master branch, yet? If so, I am fine with finally removing _hold.

[QuLogic]

No, SciTools/cartopy#1130 is waiting for @dopplershift to finish conda-forge/matplotlib-feedstock#181 so we can test things properly.

[dopplershift]

Top of the list as soon as unbury from my trip last week.

[dstansby]

Since the two issues linked by @QuLogic are merged, I figure this is good to go.

[dopplershift]

Mostly. Technically, CartoPy still needs to release those fixes, but I promise to cut that release on Friday. 😁

[tacaswell]

but I promise to cut that release on Friday. grin

Applying my usual discount rate to time estimations, next Friday ;)