Rely on SCM metatdata for version

Currently, the package version is declared statically in source code (`twine/__init__.py` or `setup.py` if #528 is accepted). That means except for the tagged releases, the `__version__` is deceiving about what version is in use (current master is "2.0.0" but has different behavior from the released 2.0.0).



Moreover, by storing the version statically in the codebase, it becomes necessary to add a commit and change the version number before any release. It's a manual step that's prone to mistakes.



If instead the project were to rely on tags SCM metadata from which to derive a version, it eliminates that manual step and allows the version to derive from the state of the repo, reflecting the version from the tag (for tagged commits), or deriving a pre/post release for other commits or a dirty checkout.

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