Update dtype usage in zarr/meta.py (#700)

* Update dtype usage in zarr/meta.py

Changes in numpy have led to failures on `mypy zarr/`
across a large number of PRs.

* Revert encode_dtype return annotation

Author: joshmoore

zarr/meta.py

@@ -6,7 +6,7 @@
 from zarr.errors import MetadataError
 from zarr.util import json_dumps, json_loads
 
-from typing import Union, Any, List, Mapping as MappingType
+from typing import cast, Union, Any, List, Mapping as MappingType
 
 ZARR_FORMAT = 2
 
@@ -75,7 +75,7 @@ def encode_array_metadata(meta: MappingType[str, Any]) -> bytes:
     return json_dumps(meta)
 
 
-def encode_dtype(d: np.dtype) -> str:
+def encode_dtype(d: np.dtype):
     if d.fields is None:
         return d.str
     else:
@@ -180,8 +180,9 @@ def encode_fill_value(v: Any, dtype: np.dtype) -> Any:
     elif dtype.kind == 'b':
         return bool(v)
     elif dtype.kind in 'c':
-        v = (encode_fill_value(v.real, dtype.type().real.dtype),
-             encode_fill_value(v.imag, dtype.type().imag.dtype))
+        c = cast(np.complex128, np.dtype(complex).type())
+        v = (encode_fill_value(v.real, c.real.dtype),
+             encode_fill_value(v.imag, c.imag.dtype))
         return v
     elif dtype.kind in 'SV':
         v = str(base64.standard_b64encode(v), 'ascii')