Add upper/lower bounds to dependencies in pyproject.toml

There is no upper/lower bound on dependencies in `pyproject.toml`: https://github.com/GoogleCloudPlatform/alloydb-python-connector/blob/3b5ea2ff8df18e94ec70adc00f17fedae4547d2b/pyproject.toml#L43-L52

We should add lower bounds and test against those to ensure that new changes are compatible with the minimum version of a dependency.

As an example, see https://github.com/googleapis/python-bigquery-storage/blob/main/testing/constraints-3.7.txt where minimum versions of dependencies are tested. The reason is that we want tests to fail if the minimum version that we specify no longer works

For the upper bound, one reason to prevent pulling the latest major version of a dependency is that it's untested and could have a breaking change.

`requirements.txt` and `constraints.txt` have different purposes. For constraints, we want to check that tests pass with the minimum versions of dependencies. `requirements.txt` helps maintainers/CI have a consistent environment when running tests. In the latter case, we may want the latest versions of dependencies. May want to consider renaming the current `requirements.txt` to `requirements-test.txt` to show it is for testing.