Fix PackageMetadata validation error with extra provided field

[aganders3]

The PackageMetadata class is configured to ignore extra fields but instantiation can still fail (raises a KeyError) when actually provided an extra field due to this custom root validator. Based on the config this does not seem like the intended behavior.

class PackageMetadata(BaseModel):
    ....
    class Config:
        extra = Extra.ignore
    ...
    @root_validator(pre=True)
    def _validate_root(cls, values):
        if "metadata_version" not in values:
            fields = cls.__fields__
            mins = {fields[n].field_info.extra.get("min_ver", "1.0") for n in values}
            values["metadata_version"] = str(max(float(x) for x in mins))
        return values

This PR just changes that validator skip over any values not in the list of expected fields during this step.

[codecov]

Codecov Report

Merging #256 (a75fb68) into main (367b31f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #256   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           37        37           
  Lines         2807      2807           
=========================================
  Hits          2807      2807           

Impacted Files	Coverage Δ
npe2/manifest/_package_metadata.py	100.00% <100.00%> (ø)

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[nclack]

Thanks!

You can remove the pyside dependency in the ci. It'll cause a test to fail, but that should get fixed in #255.