Streamline publishing process

[bisgaard-itis]

What do these changes do?

• Ensure client version comes from a VERSION file in clients/python/client.
• Add a make recipe which will generate a VERSION file. The version generated has the format <latest released version>+<number of commits from latest released commits>. This ensures every new pushed commit can be published on TestPypi with a unique version number. I have tested that packaging.version.Version behaves properly with respect to this type of versioning. E.g. Version("3.2.6+15")>Version("3.2.6+14") evaluates to True while Version("3.2.6+15")>Version("3.2.7+14") evaluates to False.
• Update the github workflow so it generates a correct VERSION file if the workflow is triggered by a tagged commit (=release) and every new commits on master is published to TestPypi.
• Clean up release workflow. Now releasing a client is done entirely through Github. Simply tag a commit with a tag of the type v3.2.4.

Related issue/s

How to test

For internal developers

[pcrespov]

🎉 thx!