use setuptools_scm for version management

pyproject.toml

@@ -0,0 +1,5 @@
+[build-system]
+requires = ["setuptools>=45", "setuptools_scm[toml]>=6.2"]
+
+[tool.setuptools_scm]
+write_to = "src/allmydata/_version.py"

src/allmydata/__init__.py

@@ -3,55 +3,14 @@ Decentralized storage grid.
 
 community web site: U{https://tahoe-lafs.org/}
 """
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
-from future.utils import PY2, PY3
-if PY2:
-    # Don't import future str() so we don't break Foolscap serialization on Python 2.
-    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, max, min  # noqa: F401
-    from past.builtins import unicode as str
 
 __all__ = [
     "__version__",
-    "full_version",
-    "branch",
     "__appname__",
-    "__full_version__",
 ]
 
-__version__ = "unknown"
-try:
-    # type ignored as it fails in CI
-    # (https://app.circleci.com/pipelines/github/tahoe-lafs/tahoe-lafs/1647/workflows/60ae95d4-abe8-492c-8a03-1ad3b9e42ed3/jobs/40972)
-    from allmydata._version import __version__  # type: ignore
-except ImportError:
-    # We're running in a tree that hasn't run update_version, and didn't
-    # come with a _version.py, so we don't know what our version is.
-    # This should not happen very often.
-    pass
-
-full_version = "unknown"
-branch = "unknown"
-try:
-    # type ignored as it fails in CI
-    # (https://app.circleci.com/pipelines/github/tahoe-lafs/tahoe-lafs/1647/workflows/60ae95d4-abe8-492c-8a03-1ad3b9e42ed3/jobs/40972)
-    from allmydata._version import full_version, branch  # type: ignore
-except ImportError:
-    # We're running in a tree that hasn't run update_version, and didn't
-    # come with a _version.py, so we don't know what our full version or
-    # branch is. This should not happen very often.
-    pass
-
 __appname__ = "tahoe-lafs"
-
+from allmydata._version import __version__
-# __full_version__ is the one that you ought to use when identifying yourself
-# in the "application" part of the Tahoe versioning scheme:
-# https://tahoe-lafs.org/trac/tahoe-lafs/wiki/Versioning
-__full_version__ = __appname__ + '/' + str(__version__)
-
 
 # Install Python 3 module locations in Python 2:
 from future import standard_library