Preparations for PEP440 support

Currrently, builds for patches are broken because the version numbers
generated for them are not valid according to PEP 440 [1].

Previous attempts [2] have shown that this cannot be solved in a single
commit (needs coordination with sr.ht-apkbuilds), so here is an attempt
at preparing this repo for a switch without breaking anything.

The grand plan is roughly:

1. Add a pyproject.toml without touching setup.py (this commit)
2. Switch APKBUILD from `python setup.py build` to `python -m build`
3. Reduce setup.py to a stub, encoding all relevant information in
   pyproject.toml

With this commit, this module can be build with both `python setup.py
build` and `python -m build`, if, _and only if_ the PKGVER environment
variable is set, which is true for all our tooling.

As the version passed in via the environment is still not
PEP440-compatible, packaging non-tagged versions will remain broken
until step three above is executed.

The .gitattributes and .git_archival files are included for the future
setup. Since packages are built from `git-archive` tarballs, the commit
information has to be transported into the tarballs. The setuptools-scm
package specifies a mechanism for this [3]. Note, that in order to avoid
a hilarious bug [4] the checked in `.git_archival.txt` differs from the
template found in the documentation. The git version on git.sr.ht is new
enough that the `describe-name` will be expanded, and if present it is
the only information that setuptools-scm really requires.

[1] https://peps.python.org/pep-0440
[2] https://lists.sr.ht/~sircmpwn/sr.ht-dev/patches/50784
[3]: https://setuptools-scm.readthedocs.io/en/latest/usage/#git-archives
[4]: https://github.com/pypa/setuptools_scm/issues/806

.git_archival.txt

@@ -0,0 +1 @@
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$

.gitattributes

@@ -0,0 +1 @@
+.git_archival.txt  export-subst

pyproject.toml

@@ -0,0 +1,58 @@
+[build-system]
+requires = ["setuptools", "setuptools_scm"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "srht"
+dynamic = ["version"]
+description = "sr.ht core modules"
+readme = "README.md"
+authors = [{name = "Drew DeVault", email = "sir@cmpwn.com"}]
+dependencies = [
+      "flask",
+      "humanize",
+      "sqlalchemy",
+      "sqlalchemy-utils",
+      "psycopg2",
+      "markdown",
+      "mistletoe",
+      "bleach",
+      "requests",
+      "BeautifulSoup4",
+      "pygments",
+      "cryptography",
+      "prometheus_client",
+      "alembic",
+      "redis",
+      "celery",
+]
+license.text = "BSD-3-Clause"
+
+[project.urls]
+repository = "https://git.sr.ht/~sircmpwn/core.sr.ht"
+
+[tool.setuptools]
+packages = [
+    "srht",
+    "srht.alembic",
+    "srht.alembic.versions",
+    "srht.graphql",
+    "srht.oauth",
+    "srht.webhook",
+]
+[tool.setuptools.package-data]
+srht = [
+    'Makefile',
+    'minify-css.js',
+    'templates/*.html',
+    'scss/*.scss',
+    'scss/*.css',
+    'scss/bootstrap/LICENSE',
+    'scss/bootstrap/scss/*.scss',
+    'scss/bootstrap/scss/mixins/*.scss',
+    'scss/bootstrap/scss/utilities/*.scss',
+    'static/*',
+    'static/icons/*',
+]
+
+[tool.setuptools_scm]