Improve SelectField enum coerce

[azmeuk]

Implementing suggestions from #922. No more implicit magic for coerce= with Enum values, SelectChoice.coerce_by_name is introduced to be used instead (coerce=SelectChoice.coerce_by_name). I am not delighted with the name but cannot find a better one.
/cc @miketheman @Daverball

edit: that damned Copilot review button...

[Daverball]

Looks good to me. I also can't think of a better name for the helper method, apart from maybe coerce_enum_by_name, to really drive home that this method is for enums, but the name also doesn't seem that bad to me in the first place, the only slightly awkward thing about it, is that SelectChoice may not be the most natural place to look for such a method, maybe it makes more sense to define it on SelectField itself?

[miketheman]

I haven't tried the code yet, but thinking about naming, how might this look?

class PaintForm(Form):
  color = SelectField(
    choices=SelectChoice.from_enum(Color),
    coerce=SelectChoice.enum_name(Color),
  )

That allows for some future enum_value that handles the other pattern.

The UX still "feels weird" overall - since I'm not asking to coerce one type into another type (like the coerce_none example in the docs), rather I'm asking the SelectField choices options of (value, label, render_kw) to be populated with some part of an enum, so maybe the overall from_enum could do it for me, instead of only allowing the label input to affect what's displayed, but also the value?

class PaintForm(Form):
  color = SelectField(
    choices=SelectChoice.from_enum(
      Color,
      label=some_callable,
      value=some_other_callable),
  ),

[azmeuk]

After giving some more thoughts I went to two independent enum_choices and enum_coerce methods with a by attribute, so either the enum label or the enum value can be used as form value.