Python: Mixing implicit/explicit returns false positive

**Description of the false positive**

I am seeing an ` Mixing implicit and explicit returns may indicate an error as implicit returns always return None.` alert where the code actually always has a return.
In my interpretation of the alert it seems that the issue is that CodeQL isn't considering the `case _:` in a match to be acting as a default case so it believes that there should be a `return None` outside of the `match`.

**Code samples or links to source code**

I had to slightly redact the code but I believe that this is the same:
(ironically this is in a piece of code that processes codeql alerts, don't get confused 😄)

```python
def codeql_severity_conversion(self, security_severity, severity):
match (security_severity, severity):
case ("critical", _) | ("high", _) | ("low", _):
return security_severity
case ("medium", _):
return "moderate"
case (None, "error"):
return "high"
case (None, "warning"):
return "moderate"
case (None, "note") | (None, "none"):
return "low"
case _:
return "high"
```

**URL to the alert on GitHub code scanning (optional)**

I'd rather not share the link outside the enterprise. The link to the rule in the alert is https://github.com/github/codeql/blob/d42788844f7ec0a6b9832140313cc2318e513987/python/ql/src/Functions/ConsistentReturns.ql

Start by reading python/ql/src/Functions/ConsistentReturns.ql and compare its return-flow handling with the supplied Python match example. Reproduce the alert if possible; done means a function with a case _ default return is no longer reported as mixing implicit and explicit returns.

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