tox: specific testenv for HDK context
In the context of the Heptapod Development Kit (HDK), we use a wrapper launch script, which gives us the opportunity to inject environment variables. Notably we can: - use the same version as Python Heptapod components (such as HGitaly - avoid the system defaults. A current use case is that pytest-parallel does not support Python 3.9 yet. With the `HDK_HEPTAPOD_TESTS_BASEPYTHON` environment variable, we can define `testenv:hdk` without local modifications to `tox.ini`. The default `python3` value should help in transition time, even if it doesn't seem likely that we'll have wrapper scripts that use the `hdk` testenv without providing the environment variable value.