Switch from versioneer to concrete version in setup.py

[simonw]

The new PyPI resolver keeps on showing me warnings like this one when I install Datasette directly from GitHub using pip install https://github.com/simonw/datasette/archive/main.zip:

Successfully built datasette
Installing collected packages: datasette
  Attempting uninstall: datasette
    Found existing installation: datasette 0.50.2
    Uninstalling datasette-0.50.2:
      Successfully uninstalled datasette-0.50.2
ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

We recommend you use --use-feature=2020-resolver to test your packages with the new resolver before it becomes the default.

datasette-upload-csvs 0.5 requires datasette>=0.47, but you'll have datasette 0+unknown which is incompatible.
datasette-publish-vercel 0.8 requires datasette>=0.44, but you'll have datasette 0+unknown which is incompatible.
datasette-psutil 0.2 requires datasette>=0.44, but you'll have datasette 0+unknown which is incompatible.
datasette-leaflet-geojson 0.6 requires datasette>=0.48, but you'll have datasette 0+unknown which is incompatible.
datasette-edit-schema 0.3 requires datasette>=0.44, but you'll have datasette 0+unknown which is incompatible.
datasette-cluster-map 0.13 requires datasette>=0.48, but you'll have datasette 0+unknown which is incompatible.
Successfully installed datasette-0+unknown

This is because we use versioneer. I'm going to drop that in favour of embedding the version directly in setup.py, like I do in other projects such as sqlite-utils.

I'll use a .dev suffix in the development version, as suggested by https://www.python.org/dev/peps/pep-0440/#developmental-releases

[simonw]

Maybe imitate how Django does this, e.g. django/django@6b9b2af

[simonw]

Essentially it's this problem: python-versioneer/python-versioneer#140