Fix some mypy errors

[jpmckinney]

Part of #217

• chore(typing): Use cast() between compatible models for type checker
• chore(typing): Re-order get_new_contracts for clarity (and avoid changing types in reassignments)

[coveralls]

Pull Request Test Coverage Report for Build 7020947513

• 2 of 7 (28.57%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 86.319%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/sources/colombia.py	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
app/sources/colombia.py	1	72.62%
app/commands.py	3	96.49%

Totals
Change from base Build 7020146929:	0.1%
Covered Lines:	2000
Relevant Lines:	2317

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💛 - Coveralls