Remove restriction on DotDict critical keys

[jlowin]

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Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

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What does this PR change?

Previously, DotDicts were restricted from setting "critical" keys (such as update, items, etc.) because they would shadow dict methods. The only difference between a GraphQLResult and a DotDict was that this restriction was lifted and a different repr.

However, in practice, this restriction is annoying and/or confusing.

This PR removes the restriction. The motivation for this PR was actually the discovery of a bug that as_nested_dict failed if a GraphQLResult had an items key. This PR fixes the bug but in fixing it, I realized the restriction was obviously unhelpful since this bug slipped by anyway.

Why is this PR important?

[codecov]

Codecov Report

Merging #1462 into master will decrease coverage by <.01%.
The diff coverage is 100%.

[cicdw]

Very clean implementation that avoids the special keys, I like it - two minor suggestions