Ensure we build sensible sdists.

.coveragerc

@@ -3,7 +3,7 @@ branch = True
 include =
   amaranth/*
 omit =
-  amaranth/test/*
+  tests/*
 
 [report]
 exclude_lines =

CONTRIBUTING.txt

@@ -4,6 +4,9 @@ We love contributions from the community. Please send pull requests and raise
 issues! All code submissions require review. We use GitHub pull requests for
 this purpose.
 
+See the "Contributing" section of the documentation manual for more details
+about contributing to Amaranth.
+
 
 COPYRIGHTS and LICENSE
 

pyproject.toml

@@ -46,6 +46,13 @@ build-backend = "pdm.backend"
 # Add a workaround to improve experience for people upgrading from old checkouts.
 includes = ["amaranth/"]
 
+source-includes = [
+  ".gitignore",
+  ".coveragerc",
+  ".env.toolchain",
+  "CONTRIBUTING.txt",
+]
+
 # Development workflow configuration
 
 [tool.pdm.dev-dependencies]