Use Marshmallow's native Enum field if possible

[dargueta]

This allows us to remove the dependency on marshmallow_enum if marshmallow 3.18+ is installed.

Changes

• Use marshmallow.fields.Enum if present (added in 3.18)
• Explicitly throw UnknownFieldError if an enum is encountered but an implementation is not available. Previously, this would throw an ImportError for marshmallow_enum which is confusing for users not familiar with this requirement.

Other changes:

• Removed unused imports
• Removed nonexistent items from __all__
• Added 3.10 and 3.11 to the trove classifiers

[codecov[bot]]

Codecov Report

Merging #260 (1032862) into main (0c43ff5) will decrease coverage by 1.46%. The diff coverage is 41.66%.

@@             Coverage Diff             @@
##              main     #260      +/-   ##
===========================================
- Coverage   100.00%   98.54%   -1.46%     
===========================================
  Files            6        6              
  Lines          472      480       +8     
  Branches        90       91       +1     
===========================================
+ Hits           472      473       +1     
- Misses           0        6       +6     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
src/desert/_make.py	94.81% <41.66%> (-5.19%)	:arrow_down:

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[dargueta]

Unless we run the test suite four times, one for each combination of Marshmallow + backport package, I don't know how we'd keep coverage at 100%. (I'm guessing pragma: no cover is off the table?)

[ChenoChosencar]

(feel free to delete if out of place) This would fix an enum field incompatibility with apispec detailed here: https://github.com/marshmallow-code/apispec/issues/506