Package Instance

PyJWT Issuer field partial matches allowed
### Summary
The wrong string if check is run for `iss` checking, resulting in `"acb"` being accepted for `"_abc_"`.

### Details
This is a bug introduced in version [2.10.0](https://github.com/jpadilla/pyjwt/commit/1570e708672aa9036bc772476beae8bfa48f4131#diff-6893ad4a1c5a36b8af3028db8c8bc3b62418149843fc382faf901eaab008e380R366): checking the "iss" claim
changed from `isinstance(issuer, list)` to `isinstance(issuer,
Sequence)`.

```diff
-        if isinstance(issuer, list):
+        if isinstance(issuer, Sequence):
            if payload["iss"] not in issuer:
                raise InvalidIssuerError("Invalid issuer")
        else:
```

Since str is a Sequnce, but not a list, `in` is also used for string
comparison. This results in `if "abc" not in "__abcd__":` being
checked instead of `if "abc" != "__abc__":`.
### PoC
Check out the unit tests added here: https://github.com/jpadilla/pyjwt-ghsa-75c5-xw7c-p5pm
```python
        issuer = "urn:expected"

        payload = {"iss": "urn:"}

        token = jwt.encode(payload, "secret")

        # decode() succeeds, even though `"urn:" != "urn:expected". No exception is raised.
        with pytest.raises(InvalidIssuerError):
            jwt.decode(token, "secret", issuer=issuer, algorithms=["HS256"])
```


### Impact

I would say the real world impact is not that high, seeing as the signature still has to match. We should still fix it.