fix: Correctly catch JSONDecodeError exceptions (#26)

Problem:

Since `requests` version `2.27.0` [1], the library has introduced its
own `JSONDecodeError` exception, to fix inconsistencies between `json`
and `simplejson`.

From that version onwards, if the project has `simplejson` installed,
the exception raised by `json()` is a subclass of
`simplejson.JSONDecodeError` instead of `json.JSONDecodeError`.

The Knock client is only handling `json.JSONDecodeError`, so in projects
where `simplejson` is installed, the exception is raised when `json()`
is not able to decode the response body (e.g. in 204 responses).

How to reproduce:

In a new virtualenv, install `requests`.

```python
>>> import json, requests
>>> isinstance(requests.exceptions.JSONDecodeError("", "", 0), json.JSONDecodeError)
True
```

Now, install `simplejson`.

```python
>>> import json, requests, simplejson
>>> isinstance(requests.exceptions.JSONDecodeError("", "", 0), json.JSONDecodeError)
False
>>> isinstance(requests.exceptions.JSONDecodeError("", "", 0), simplejson.JSONDecodeError)
True
```

Solution:

When available, import the `JSONDecodeError` class provided by
`requests`, and use that one to handle exceptions from the `json()`
method. Otherwise, first try to use the exception provided by
`simplejson`, and only use the one from `json` if none is available.

[1] psf/requests@db575ee

Co-authored-by: Chris Bell <cjbell1988@gmail.com>

Author: adamantike

knockapi/client.py

@@ -1,7 +1,15 @@
 import requests
-from json.decoder import JSONDecodeError
 from knockapi.__about__ import __version__
 
+try:
+    from requests.exceptions import JSONDecodeError
+except ImportError:
+    try:
+        from simplejson import JSONDecodeError
+    except ImportError:
+        from json.decoder import JSONDecodeError
+
+from ._version import __version__
 
 class Connection(object):
     def __init__(self, api_key, host='https://api.knock.app', timeout=None):