text: Title: Not able to index when using List[str] in custom document class
  Body: System: `macOS 13.3.1`
  Python version: `3.11.0`
  IPython version: `8.10.0`
  
  In the latest docarray version, when building hnsw index from the following simple document class:
  ```python
  from docarray import BaseDoc
  from docarray.index import HnswDocumentIndex
  
  class MyDoc(BaseDoc):
      test: List[str]
  
  index = HnswDocumentIndex[MyDoc]()
  ```
  The following error will pop up:
  ```
  File ~/miniconda3/envs/test/lib/python3.11/site-packages/docarray/index/backends/hnswlib.py:76, in HnswDocumentIndex.__init__(self, db_config, **kwargs)
       73 if db_config is not None and getattr(db_config, 'index_name'):
       74     db_config.work_dir = db_config.index_name.replace("__", "/")
  ---> 76 super().__init__(db_config=db_config, **kwargs)
       77 self._db_config = cast(HnswDocumentIndex.DBConfig, self._db_config)
       78 self._work_dir = self._db_config.work_dir
  
  File ~/miniconda3/envs/test/lib/python3.11/site-packages/docarray/index/abstract.py:111, in BaseDocIndex.__init__(self, db_config, subindex, **kwargs)
      109 self._runtime_config = self.RuntimeConfig()
      110 self._logger.info('Runtime config created')
  --> 111 self._column_infos: Dict[str, _ColumnInfo] = self._create_column_infos(
      112     self._schema
      113 )
      114 self._is_subindex = subindex
      115 self._subindices: Dict[str, BaseDocIndex] = {}
  
  File ~/miniconda3/envs/test/lib/python3.11/site-packages/docarray/index/abstract.py:880, in BaseDocIndex._create_column_infos(self, schema)
      873 """Collects information about every column that is implied by a given schema.
      874
      875 :param schema: The schema (subclass of BaseDoc) to analyze and parse
      876     columns from
      877 :returns: A dictionary mapping from column names to column information.
      878 """
      879 column_infos: Dict[str, _ColumnInfo] = dict()
  --> 880 for field_name, type_, field_ in self._flatten_schema(schema):
      881     # Union types are handle in _flatten_schema
      882     if issubclass(type_, AnyDocArray):
      883         column_infos[field_name] = _ColumnInfo(
      884             docarray_type=type_, db_type=None, config=dict(), n_dim=None
      885         )
  
  File ~/miniconda3/envs/test/lib/python3.11/site-packages/docarray/index/abstract.py:860, in BaseDocIndex._flatten_schema(cls, schema, name_prefix)
      856     else:
      857         raise ValueError(
      858             f'Union type {t_} is not supported. Only Union of subclasses of AbstractTensor or Union[type, None] are supported.'
      859         )
  --> 860 elif issubclass(t_, BaseDoc):
      861     names_types_fields.extend(
      862         cls._flatten_schema(t_, name_prefix=inner_prefix)
      863     )
      864 elif issubclass(t_, AbstractTensor):
  
  File <frozen abc>:123, in __subclasscheck__(cls, subclass)
  
  TypeError: issubclass() arg 1 must be a class
  ```
  
  This also happens in other typings like Sequence, Iterable and Tuple.