fix: Use tuples for pkey check

[SpaceCondor]

https://github.com/MeltanoLabs/target-postgres/blob/d07b41583e8ff77ee770a0d40779ea9485772461/target_postgres/sinks.py#L161-L169

Currently the code above uses string concatenation to check for duplicate primary key values, however this is problematic since the records below will be treated as the same record and the first will be omitted:

Record 1:

• Primary Key 1: AB
• Primary Key 2: C

Record 2:

• Primary Key 1: A
• Primary Key 2: BC

Changing to a tuple should mitigate this.

[edgarrmondragon]

~hmm tests are no longer running on PRs from forks...~

Nevermind: https://github.com/MeltanoLabs/target-postgres/pull/447

[edgarrmondragon]

Thanks @SpaceCondor!