How to specify that headers must be regenerated.

I've have a GitHub app which uses a jwt that can expire. Ultimately, I generate an installation access token that can also expire and I am trying to use `ghapi` with that installation access token.

I was hoping that I could subclass `GhApi.__call__` to dynamically generate my access token, but this does not seem to work.

```python
jwtm = JwtManager(app_id, private_key)
iatm = IatManager(jwtm)
token = iatm.token(installation_id)
gh_host = baseurl()

class GhApi(ghapi_all.GhApi):
def __call__(self, path, verb=None, headers=None, route=None, query=None, data=None):
headers = headers or {}
headers.update(iatm.headers(installation_id))
super().__call__(path, verb, headers, route, query, data)

api = GhApi(
owner=owner,
repo=repo,
token=token,
debug=debug,
limit_cb=limit_cb,
gh_host=gh_host,
)
```

IatManager.headers() will generate a new jwt token whenever it is expired and then will use this to generate an access token for the specified installation id. The headers it generates look like this:
```python
In [5]: iatm.headers(2)
Out[5]:
{'Authorization': 'token ghs_randomstuffhere',
'Accept': 'application/vnd.github.v3+json'}
```
The reason that this did not work is that ultimately `GhApi.__call__` is not what was called. Rather it was some lower level thing. Any suggestion on how I can use `ghapi` for a long-running GitHub app where the headers need to be dynamically updated in time?