Fix KeyError when delete required property from schema with patch method

[guimunarolo]

Description

Improve delete of required property from OpenAPI schema avoiding KeyError exception.

Traceback:

ERROR    django.request:log.py:228 Internal Server Error: /
Traceback (most recent call last):
  File "/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/lib/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/lib/python3.7/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/lib/python3.7/site-packages/rest_framework/views.py", line 505, in dispatch
    response = self.handle_exception(exc)
  File "/lib/python3.7/site-packages/rest_framework/schemas/views.py", line 48, in handle_exception
    return super().handle_exception(exc)
  File "/lib/python3.7/site-packages/rest_framework/views.py", line 465, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/lib/python3.7/site-packages/rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "/lib/python3.7/site-packages/rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "/lib/python3.7/site-packages/rest_framework/schemas/views.py", line 37, in get
    schema = self.schema_generator.get_schema(request, self.public)
  File "/lib/python3.7/site-packages/rest_framework/schemas/openapi.py", line 64, in get_schema
    paths = self.get_paths(None if public else request)
  File "/lib/python3.7/site-packages/rest_framework/schemas/openapi.py", line 47, in get_paths
    operation = view.schema.get_operation(path, method)
  File "/lib/python3.7/site-packages/rest_framework/schemas/openapi.py", line 101, in get_operation
    request_body = self._get_request_body(path, method)
  File "/lib/python3.7/site-packages/rest_framework/schemas/openapi.py", line 466, in _get_request_body
    del content['required']
KeyError: 'required'

[carltongibson]

Yep, super. Thanks!