Preempt issubclass error for generic types

[msuozzo]

Previously, parameterized generics that made it to the issubclass call would
fail and cause obscure stack traces to bubble up. Short-circuiting this logic
by unconditionally rejecting ensures the type check failure case will handle
the error message generation and display.

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[msuozzo]

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R: @damccorm for label python.

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[damccorm]

R: @jrmccluskey

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[jrmccluskey]

LGTM, thank you!