JSONDecodeError not handled in _request() method

[nanda-mik]

My team uses postgrid-python(v1.0.14) to integrate with postgrid and send letter. Recently encountered an error for this which is not handled in the exception catch and led to unexpected failure. We are handling only PMError & NotImplementedError from your side, but got JSONDecodeError which breaks our flow. So in your code it should be handled in line no 139.

Code snippet

def _request(endpoint, method="GET", idempotency_key=None, **kwargs):
    .......
    ......

        for key, value in kwargs.items():
            files = flatten(files, data, key, value)
        res = requests.post(pm_base + endpoint, data=data, files=files, headers=headers)
    elif method == "DELETE":
        res = requests.delete(pm_base + endpoint, params=kwargs, headers=headers)
    else:
        raise NotImplementedError()

    value = res.json() -> This line needs to be inside try-catch...

    try:
    ........
    .......
    ```

Attaching the stack track below:
**Error trace**
    File "/home/appuser/src/CommunicationService/utils/postgrid_letter_backend.py", line 112, in send_letters
letter_obj = postgrid.Letter.create(
File "/usr/local/lib/python3.9/site-packages/postgrid/__init__.py", line 342, in create
return super().create(locals())
File "/usr/local/lib/python3.9/site-packages/postgrid/__init__.py", line 190, in create
return _pm_post(cls.endpoint.format(parent_resource_id), **locals_except_kwargs_and_cls, **locals_['kwargs'])
File "/usr/local/lib/python3.9/site-packages/postgrid/__init__.py", line 163, in _pm_post
return _request(pm_base, endpoint, method='POST', **kwargs)
File "/usr/local/lib/python3.9/site-packages/postgrid/__init__.py", line 139, in _request
value = res.json()
File "/usr/local/lib/python3.9/site-packages/requests/models.py", line 975, in json
raise RequestsJSONDecodeError(e.msg, e.doc, e.pos)
requests.exceptions.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Can i give you a PR to fix this and work on your codebase? Please let me know about it.    

[goodpaul6]

@nanda-mik do you happen to have the request that failed with this error? In general, our API should not respond with anything but JSON.