pyproject: migrate to PDM build backend.

`.git_archival.txt` and the functionality of deriving SCM version
from a GitHub generated archive is removed pending implementation of
pdm-project/pdm-backend#194.

.gitattributes

@@ -1,2 +1 @@
-.git_archival.txt export-subst
 /amaranth/vendor/* -linguist-vendored

pdm_build.py

@@ -0,0 +1,13 @@
+from pdm.backend._vendor.packaging.version import Version
+
+
+# This is done in a PDM build hook without specifying `dynamic = [..., "version"]` to put all
+# of the static metadata into pyproject.toml. Tools other than PDM will not execute this script
+# and will use the generic version of the documentation URL (which redirects to /latest).
+def pdm_build_initialize(context):
+    version = Version(context.config.metadata["version"])
+    if version.is_prerelease:
+        url_version = "latest"
+    else:
+        url_version = f"v{version}"
+    context.config.metadata["urls"]["Documentation"] += url_version

pyproject.toml

@@ -1,7 +1,10 @@
 # Project metadata
 
+[tool.pdm.version]
+source = "scm"
+
 [project]
-dynamic = ["version", "urls"]
+dynamic = ["version"]
 
 name = "amaranth"
 description = "Amaranth hardware definition language"
@@ -23,21 +26,24 @@ remote-build  = ["paramiko~=2.7"]
 [project.scripts]
 amaranth-rpc = "amaranth.rpc:main"
 
+[project.urls]
+"Homepage" = "https://amaranth-lang.org/"
+"Documentation" = "https://amaranth-lang.org/docs/amaranth/" # modified in pdm_build.py
+"Source Code" = "https://github.com/amaranth-lang/amaranth"
+"Bug Tracker" = "https://github.com/amaranth-lang/amaranth/issues"
+
 # Build system configuration
 
 [build-system]
-requires = ["wheel", "setuptools>=67.0", "setuptools_scm[toml]>=6.2"]
-build-backend = "setuptools.build_meta"
+requires = ["pdm-backend"]
+build-backend = "pdm.backend"
 
-[tool.setuptools]
+[tool.pdm.build]
 # If amaranth 0.3 is checked out with git (e.g. as a part of a persistent editable install or
 # a git worktree cached by tools like poetry), it can have an empty `nmigen` directory left over,
 # which causes a hard error because setuptools cannot determine the top-level package.
 # Add a workaround to improve experience for people upgrading from old checkouts.
-packages = ["amaranth"]
-
-[tool.setuptools_scm]
-local_scheme = "node-and-timestamp"
+includes = ["amaranth/"]
 
 # Development workflow configuration