Stop tests from being bundled with package

[finlaysawyer]

Currently pynamodb-attributes is bundling its test suite into site-packages, which causes problems with build tooling such as Bazel where it's treating tests as a third party import and conflicting with our own test suite.

(venv) ➜  /tmp ls -l ./venv/lib/python3.12/site-packages/
total 72
...
drwxr-xr-x  19 finlaysawyer  wheel    608 Sep 18 18:07 pynamodb
drwxr-xr-x   8 finlaysawyer  wheel    256 Sep 18 18:07 pynamodb-6.0.1.dist-info
drwxr-xr-x  17 finlaysawyer  wheel    544 Sep 18 18:07 pynamodb_attributes
drwxr-xr-x   9 finlaysawyer  wheel    288 Sep 18 18:07 pynamodb_attributes-0.5.0.dist-info
drwxr-xr-x  19 finlaysawyer  wheel    608 Sep 18 18:07 tests <--------