Support accessing reserved word resource properties via attribute

[anniel-stripe]

Currently, accessing a property with the same name as a Python reserved word as a resource attribute does not work. The only known case of this is for the country code is on country_options of a Tax Registration resource.

Accessing the property as an object attribute will result in a syntax error:

>>> taxreg.country_options.is
  File "<stdin>", line 1
    reg.country_options.is
                        ^^
SyntaxError: invalid syntax

This means the nested property can only be accessed via dictionary key: reg.country_options["is"].

_field_remappings allows reserved word properties to be accessed as an attribute via the property name + _, like so:

>>> taxreg.country_options.is_
<Is at 0x104b5ef70> JSON: {
  "type": "standard"
}

The _field_remappings property is already generated in stripe/tax/_registration.py.