[py-tx] Typing of SignalExchangeAPIWithSimpleUpdates is too Generic | remove use of t.Any

[BarrettOlson]

See #1254

In order pass mypy the last arg in the type of
SignalExchangeAPIWithSimpleUpdates was made generic.

Previous value: state.TFetchedSignalMetadata which gave error
threatexchange/exchanges/signal_exchange_api.py:269: error: Variance of TypeVar "state.TFetchedSignalMetadata" incompatible with variance in parent type [type-var]

Having this be generic is risky because SignalExchangeAPIWithSimpleUpdates is one of the single most important interface in pytx

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Notes from brief attempt to fix: When Barrett attempted to replace value with state.TUpdateRecordValue mypy broke further along in files using SignalExchangeAPIWithSimpleUpdates so there may more bugs hidden behind this issue already.