Generate version string dynamically

[Pennycook]

Related issues

• Motivated by Add --debug option #177, since version string is now in the debug log.

Proposed changes

• Use importlib to extract the version metadata from whatever pip labelled the package when it was installed.
• Configure setuptools-scm to generate the version string automatically from the latest semver tag and git status.
• Move codebasin.version to codebasin.__version__ to align with the convention used by other packages.
• Fix cbi-tree --version, which used the string "CBI Coverage Tool" by mistake.

---

The new behavior in action:

>>> import codebasin
>>> codebasin.__version__
'1.2.1.dev313+g63df3e8'

$ codebasin --version
Code Base Investigator 1.2.1.dev313+g63df3e8

$ cbi-cov --version
CBI Coverage Tool 1.2.1.dev313+g63df3e8

$ cbi-tree --version
CBI Tree Tool 1.2.1.dev313+g63df3e8

[Pennycook]

Failing coverage tests are just for checking the debug string, and I'm not sure we can really test that meaningfully.

[laserkelvin]

Still shocked it was straightforward!