prototype Pydantic choice support in create_pydantic_model

[dantownsend]

Related to #467

Just a prototype at the moment.

If a Piccolo column has choices, we can use the choices Enum as the Pydantic type. For example:

class RelationshipType(str, enum.Enum):
    colleague = "colleague"
    friend = "friend"

class Person(Table):
    type = Varchar(choices=RelationshipType)    

It would become this Pydantic model:

class PersonModel(BaseModel):
    type: RelationshipType

The problem is the JSON schema it generates is quite complex, and Piccolo Admin doesn't currently understand $ref values:

So the options are:

1. Modify Piccolo Admin so it understands $ref
2. Start work on PydanticModel class #331, which would add a new API for creating Pydantic models. create_pydantic_model will still be supported, and used by Piccolo admin for now, but we can migrate to this new API at a later date.

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Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (52f01ae) to head (ce4f905).
⚠️ Report is 459 commits behind head on master.

Files with missing lines	Patch %	Lines
piccolo/utils/pydantic.py	75.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #469      +/-   ##
==========================================
- Coverage   90.84%   90.83%   -0.02%     
==========================================
  Files         104      104              
  Lines        6913     6915       +2     
==========================================
+ Hits         6280     6281       +1     
- Misses        633      634       +1     

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[sinisaos]

@dantownsend Strings work well. Adding these two lines code becomes compatible with Piccolo Admin (with field propertyTitle).

# pydantic.py line 242
if column._meta.choices:
    params["title"] = column._meta.choices.__name__

In Piccolo Admin change this line to v-bind:type="property.type" and this line to class Gender(str, enum.Enum): and everything work well in admin and openapi docs.

Unfortunately ints doesn't work due to validation error.