fix: wheel includes tests

[betaboon]

Currently the wheels include tests. (see attached log 1)

This can cause problems with buildsystems like bazel.

Based on the configuration in setup.py I'm assuming this is unintended behavior.

This PR excludes them from the build. (see attached log 2)

log 1:

$ wget https://files.pythonhosted.org/packages/90/80/bebe9a4e68cf23aa4ae70bced2a36186a5fc74ff49
df4eaa70b9be163dc1/amqp_mock-0.6.1-py3-none-any.whl
$ unzip -t amqp_mock-0.6.1-py3-none-any.whl | grep -e tests
    testing: tests/_test_utils/__init__.py   OK
    testing: tests/_test_utils/amqp_client.py   OK
    testing: tests/_test_utils/fixtures.py   OK
    testing: tests/_test_utils/helpers.py   OK
    testing: tests/_test_utils/schemas.py   OK
    testing: tests/_test_utils/steps.py   OK

log 2:

$ make build
$ unzip -t dist/amqp_mock-0.6.1-py3-none-any.whl | grep -e tests

[tsv1]

Thank you for identifying and resolving the issue! Your fix has been published on PyPI as version 0.6.2