Add Optional[Dict] annotation to request_kwargs

  - Code review for ioos#280

erddapy/core/interfaces.py

@@ -5,7 +5,7 @@
 XArray, Iris, etc. objects.
 """
 
-from typing import Dict
+from typing import Dict, Optional
 
 import iris
 import pandas as pd
@@ -16,7 +16,7 @@
 from erddapy.core.url import urlopen
 
 
-def to_pandas(url: str, requests_kwargs: Dict = None, **kw) -> pd.DataFrame:
+def to_pandas(url: str, requests_kwargs: Optional[Dict] = None, **kw) -> pd.DataFrame:
     """
     Convert a URL to Pandas DataFrame.
 
@@ -51,7 +51,7 @@ def to_ncCF(url: str, protocol: str = None, **kw) -> Dataset:
 
 
 def to_xarray(
-    url: str, response="opendap", requests_kwargs: Dict = None, **kw
+    url: str, response="opendap", requests_kwargs: Optional[Dict] = None, **kw
 ) -> xr.Dataset:
     """
     Convert a URL to an xarray dataset.
@@ -71,7 +71,7 @@ def to_xarray(
         return xr.open_dataset(xr.backends.NetCDF4DataStore(nc), **kw)
 
 
-def to_iris(url: str, requests_kwargs: Dict = None, **kw):
+def to_iris(url: str, requests_kwargs: Optional[Dict] = None, **kw):
     """
     Convert a URL to an iris CubeList.