Add orm.fields.RequiredTextField, etc. for fields which cannot be null

[mesozoic]

We have several use cases where we want to

1. ensure we do not store a null value to Airtable,
2. squawk loudly if a field value in Airtable happens to be null, and
3. assume a non-null value for type checking.

Our code winds up being peppered with "if value is None, do something, else ..." conditions, and it would be easier if we could declare that as a type checked property of the field. This branch adds support for that.

For example, given this code:

from pyairtable.orm import Model, fields as F

class User(Model):
    class Meta:
        ...
    user_id = F.RequiredNumberField("ID")

The following will all raise an exception:

>>> User(user_id=None)
Traceback (most recent call last):
  ...
MissingValue: User.user_id does not accept empty values

>>> r = User.from_record(fake_record(ID=123))
>>> r.user_id
123
>>> r.user_id = None
Traceback (most recent call last):
  ...
MissingValue: User.user_id does not accept empty values

>>> r = User.from_record(fake_record(ID=None))
>>> r.user_id
Traceback (most recent call last):
  ...
MissingValue: User.user_id received an empty value

It would have been simpler to implement this as a flag on the field's constructor, but with that approach I couldn't find a way to get mypy to understand that the type of r.user_id is int and not Optional[int].

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1040512) to head (e969df6).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #363   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           24        24           
  Lines         2379      2406   +27     
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+ Hits          2379      2406   +27     

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