testsuite: Don't install BuildStream's Python dependencies

With buildstream!1027 (merged), we will no longer need to install BuildStream's Python dependencies in the testsuite images, as they will be installed on the fly when running tests.

Instead, we will need to provide the non-Python build dependencies for some of our Python dependencies that need to be built from source.

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TODO:

• Wait for consensus on buildstream!1027 (merged).