Can't handle .. paths + ref path resolution properly

[dbushong]

If you run jsonschema-gentypes with a path like --json=../path/to/some.json, and that file has relative $ref entries in it, resolution will fail.

Repro:

$ cat base.json
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$ref": "sub.json"
}
$ cat sub.json
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type":"object",
  "properties": { "x": { "type": "string" } }
}
$ jsonschema-gentypes --json=base.json --python=out.py
Processing base.json
$ cd subdir
$ jsonschema-gentypes --json=../base.json --python=out.py
Processing ../base.json
Traceback (most recent call last):
  File "/Users/dbushong/.pyenv/versions/3.8.14/lib/python3.8/site-packages/referencing/_core.py", line 417, in get_or_retrieve
    resource = registry._retrieve(uri)
  File "/Users/dbushong/.pyenv/versions/3.8.14/lib/python3.8/site-packages/jsonschema_gentypes/resolver.py", line 83, in _open_uri_resolver
    my_resource = referencing.Resource.from_contents(_open_uri(uri))
  File "/Users/dbushong/.pyenv/versions/3.8.14/lib/python3.8/site-packages/jsonschema_gentypes/resolver.py", line 63, in _open_uri
    with open(uri, encoding="utf-8") as open_file:
FileNotFoundError: [Errno 2] No such file or directory: 'sub.json'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/dbushong/.pyenv/versions/3.8.14/lib/python3.8/site-packages/referencing/_core.py", line 667, in lookup
    retrieved = self._registry.get_or_retrieve(uri)
  File "/Users/dbushong/.pyenv/versions/3.8.14/lib/python3.8/site-packages/referencing/_core.py", line 424, in get_or_retrieve
    raise exceptions.Unretrievable(ref=uri) from error
referencing.exceptions.Unretrievable: 'sub.json'

Note that it works fine if you don't have .. in your path