if a key is missing, report with a KeyError in AttrDict

[aerorahul]

Description

When a key is missing in a AttrDict, raise a KeyError consistent with dict.

Fixes #14

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• [X] pynorms
• [X] pytests

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] My changes need updates to the documentation. I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] New and existing tests pass with my changes
• [ ] Any dependent changes have been merged and published

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49.08%. Comparing base (dd9ca24) to head (91903a1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #16 +/- ## =========================================== + Coverage 48.87% 49.08% +0.21% =========================================== Files 15 15 Lines 1416 1416 Branches 308 308 =========================================== + Hits 692 695 +3 + Misses 670 667 -3 Partials 54 54 ```

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