[BUG][PYTHON] Do not set Content-Type for GET, HEAD or DELETE requests

The Python generator no longer sets a default `Content-Type` of
`application/json` for `GET`, `HEAD` and `DELETE` requests.

Having the `Content-Type` set for these requests was causing issues with
other tools which insist that GET, HEAD and DELETE requests do not have
a Content-Type (as per the OpenAPI 3 specification).

An example of the problem that this commit fixes is when using
[Prism][1] as a [validation proxy][2].

[Prism rejects any GET request that has a Content-Type][3].

Here is [an example of the problem manifesting itself][4].

To validate the fix in this commit:

1. Start with any OpenAPI3 spec e.g. the Petstore example at
https://editor.swagger.io/
2. Generate Python client code for the spec
3. Look at the generated `rest.py` e.g. in the [standard sample in this
repo][5] and see that the `Content-Type` defaults to `application/json`
for all HTTP methods (including `GET`, `HEAD` and `DELETE`), rather than
there being no `Content-Type` for `GET`, `HEAD` and `DELETE`.

Fixes OpenAPITools#9831

[1]: https://github.com/stoplightio/prism
[2]: https://meta.stoplight.io/docs/prism/docs/guides/03-validation-proxy.md
[3]: stoplightio/prism#1408 (comment)
[4]: https://github.com/agilepathway/gauge-openapi-example/pull/28/checks?check_run_id=2888606052#step:13:18
[5]: https://github.com/OpenAPITools/openapi-generator/blob/969cea8ce10cb9d012c3936fb377d631c0d044c9/samples/openapi3/client/petstore/python/petstore_api/rest.py#L141

modules/openapi-generator/src/main/resources/python/rest.mustache

@@ -129,12 +129,12 @@ class RESTClientObject(object):
                 timeout = urllib3.Timeout(
                     connect=_request_timeout[0], read=_request_timeout[1])
 
-        if 'Content-Type' not in headers:
-            headers['Content-Type'] = 'application/json'
-
         try:
             # For `POST`, `PUT`, `PATCH`, `OPTIONS`, `DELETE`
             if method in ['POST', 'PUT', 'PATCH', 'OPTIONS', 'DELETE']:
+                # Only set a default Content-Type for POST, PUT, PATCH and OPTIONS requests
+                if (method != 'DELETE') and ('Content-Type' not in headers):
+                    headers['Content-Type'] = 'application/json'
                 if query_params:
                     url += '?' + urlencode(query_params)
                 if re.search('json', headers['Content-Type'], re.IGNORECASE):