Replace versioneer with hand-rolled version generation code.

The version of versioneer has an issue that the tarball that is generated on
github changes, when the branches and/or tags refering to a commit change. This
causes issues for gridsync, which pins the hashes of those tarballs.

In addition to that, versioneer doesn't provide a meaningful revision for
untagged commits, when run from a github tarball.

Given that we only ever consume a github tarball as a github archive (except in
local development, where the version number is less important), this replaces
versioneer with some hand-rolled code specialized to that use case.

pyproject.toml

+[build-system]
+requires = ["setuptools", "wheel", "six"]
+build-backend = "setuptools.build_meta"
+
 [tool.towncrier]
     package = "_zkapauthorizer"
     package_dir = "src"

setup.cfg

@@ -60,11 +60,3 @@ install_requires =
 
 [options.extras_require]
 test = coverage; fixtures; testtools; hypothesis
-
-[versioneer]
-VCS = git
-style = pep440
-versionfile_source = src/_zkapauthorizer/_version.py
-versionfile_build = _zkapauthorizer/_version.py
-tag_prefix = release-
-parentdir_prefix = ZKAPAuthorizer

setup.py

+import os
+from runpy import run_path
+
 from setuptools import setup
 
-import versioneer
+
+def get_version():
+    """
+    Get the version of the version as determined by
+    :py:`_zkapauthorizer._version`.
+
+    Note: This only works when run from an tree generated
+    by git-archive (such as a tarball from github).
+    """
+    version_path = os.path.join(
+        os.path.dirname(__file__), "src/_zkapauthorizer/_version.py"
+    )
+    context = run_path(version_path)
+    return context["__version__"]
+
 
 setup(
-    version=versioneer.get_version(),
-    cmdclass=versioneer.get_cmdclass(),
+    version=get_version(),
     package_data={
         "": ["testing-signing.key"],
     },

src/_zkapauthorizer/__init__.py

@@ -14,9 +14,7 @@
 
 __all__ = [
     "__version__",
+    "__revision__",
 ]
 
-from ._version import get_versions
-
-__version__ = get_versions()["version"]
-del get_versions
+from ._version import __revision__, __version__