Cannot produce schema containing Literal

[layday]

Bug

Thought I'd give the new Literal a whirl and stumbled on this:

In [1]: import sys

In [2]: sys.platform, sys.version
Out[2]:
('darwin',
 '3.7.2 (default, Mar  1 2019, 12:15:59) \n[Clang 5.0.2 (tags/RELEASE_502/final)]')

In [3]: import pydantic

In [4]: pydantic.VERSION
Out[4]: StrictVersion ('0.30')

In [5]: from typing_extensions import Literal

In [6]: class Foo(pydantic.BaseModel):
   ...:     bar: Literal['baz']
   ...:

In [7]: Foo.schema()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-0d83fa816939> in <module>
----> 1 Foo.schema()

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/main.py in schema(cls, by_alias)
    461         if cached is not None:
    462             return cached
--> 463         s = model_schema(cls, by_alias=by_alias)
    464         cls._schema_cache[by_alias] = s
    465         return s

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in model_schema(model, by_alias, ref_prefix)
    233     model_name = model_name_map[model]
    234     m_schema, m_definitions, nested_models = model_process_schema(
--> 235         model, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix
    236     )
    237     if model_name in nested_models:

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in model_process_schema(model, by_alias, model_name_map, ref_prefix, known_models)
    551     known_models.add(model)
    552     m_schema, m_definitions, nested_models = model_type_schema(
--> 553         model, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix, known_models=known_models
    554     )
    555     s.update(m_schema)

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in model_type_schema(model, by_alias, model_name_map, ref_prefix, known_models)
    579         try:
    580             f_schema, f_definitions, f_nested_models = field_schema(
--> 581                 f, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix, known_models=known_models
    582             )
    583         except SkipField as skip:

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in field_schema(field, by_alias, model_name_map, ref_prefix, known_models)
    292         schema_overrides=schema_overrides,
    293         ref_prefix=ref_prefix,
--> 294         known_models=known_models or set(),
    295     )
    296     # $ref will only be returned when there are no schema_overrides

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in field_type_schema(field, by_alias, model_name_map, schema_overrides, ref_prefix, known_models)
    522             schema_overrides=schema_overrides,
    523             ref_prefix=ref_prefix,
--> 524             known_models=known_models,
    525         )
    526         definitions.update(f_definitions)

~/code/xyz/.venv/lib/python3.7/site-packages/pydantic/schema.py in field_singleton_schema(field, by_alias, model_name_map, schema_overrides, ref_prefix, known_models)
    743     if field.schema is not None and field.schema.const:
    744         f_schema['const'] = field.default
--> 745     if issubclass(field.type_, Enum):
    746         f_schema.update({'enum': [item.value for item in field.type_]})
    747         # Don't return immediately, to allow adding specific types

TypeError: issubclass() arg 1 must be a class

[dmontagu]

This same issue comes up for NewType as well. I'll take a look at this tonight.