pyproject: import coverage.py config (pypi#14758)

* pyproject: import coverage.py config

On Python 3.11+, coverage.py natively supports
pyproject.toml without needing the `toml` extra.

* pyproject: syntax

pyproject.toml

@@ -1,3 +1,33 @@
+[tool.coverage.run]
+branch = true
+dynamic_context = "test_function"
+source = ["warehouse"]
+omit = [
+  # We don't want to get coverage information for our migrations.
+  "warehouse/migrations/*",
+
+  # We don't want to actually cover our __main__.py file because it is hard
+  # to cover and it really just acts as a tiny shim to a function.
+  "warehouse/__main__.py",
+
+  # Again, tiny shim code that we don't actually need to test and trying to
+  # do so would just get in the way.
+  "warehouse/wsgi.py",
+
+  # And again, tiny shim code.
+  "warehouse/celery.py",
+]
+
+[tool.coverage.report]
+exclude_lines = [
+  "pragma: no cover",
+  "class \\w+\\(Interface\\):",
+  "if (typing\\.)?TYPE_CHECKING:",
+]
+
+# Don't show us anything that's already 100% covered.
+skip_covered = true
+
 [tool.curlylint]
 include = '\.(html|jinja|txt)$'
 # For jinja's i18n extension: