feat: Update Singer SDK version

[ReubenFrankel]

Update to the last Python 3.8-supported SDK version. The upper-Python version constrain can then be dropped (i.e. ^3.8).

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Closes #28
Supersedes #35

[ReubenFrankel]

Anything else worth getting in from the newer SDK here?

[edgarrmondragon]

Anything else worth getting in from the newer SDK here?

1. Might be worth adding dynamic_catalog: bool = True to the tap class:

   tap-mongodb/tap_mongodb/tap.py

   Line 20 in 8b5aba7

   name = "tap-mongodb"

   from meltano/sdk@ecdfa0e

2. Maybe emitting custom metrics that are appropriate for this tap, but that's probably out of scope and better to leave if to its own issue and PR.

[edgarrmondragon]

Thanks!