pip: PipProvider._known_depths sets depths to math.inf incorrectly

Description

While investigating #10201 using the following requirements:

pytest-cov
coverage==4.5.4
boto3

I printed out the inferred_depth in the method PipProvider.get_preference and noticed that almost all the packages, other than those listed above, had their inferred_depth calculated as math.inf.

Expected behavior

The actual inferred depth should be calculated. This can be fixed by moving the line to after the try/except statement:

self._known_depths[identifier] = inferred_depth

E.g. changing the code to:

try:
    requested_order: Union[int, float] = self._user_requested[identifier]
except KeyError:
    requested_order = math.inf
    parent_depths = (
        self._known_depths[parent.name] if parent is not None else 0.0
        for _, parent in information[identifier]
    )
    inferred_depth = min(d for d in parent_depths) + 1.0
else:
    inferred_depth = 1.0

self._known_depths[identifier] = inferred_depth

pip version

21.2.4

Python version

all

OS

all

How to Reproduce

1. Run pip install/download on the above requirements text

Output

No response

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