Use "source" as python-version for sdist uploads

changelog/1191.misc.txt

@@ -0,0 +1,3 @@
+- Use ``"source"`` instead of ``None`` as ``pyversion`` for ``sdist``
+  uploads. This is what PyPI (and most likely other package indexes)
+  expects.

tests/test_package.py

@@ -466,3 +466,9 @@ def test_malformed_from_file(monkeypatch):
 def test_package_from_egg():
     filename = "tests/fixtures/twine-3.3.0-py3.9.egg"
     package_file.PackageFile.from_filename(filename, comment=None)
+
+
+def test_package_from_sdist():
+    filename = "tests/fixtures/twine-1.5.0.tar.gz"
+    p = package_file.PackageFile.from_filename(filename, comment=None)
+    assert p.python_version == "source"

twine/package.py

@@ -157,16 +157,18 @@ def from_filename(cls, filename: str, comment: Optional[str]) -> "PackageFile":
                 )
             raise exceptions.InvalidDistribution(msg)
 
-        py_version: Optional[str]
         if dtype == "bdist_egg":
             (dist,) = importlib_metadata.Distribution.discover(path=[filename])
             py_version = dist.metadata["Version"]
         elif dtype == "bdist_wheel":
             py_version = cast(wheel.Wheel, meta).py_version
         elif dtype == "bdist_wininst":
             py_version = cast(wininst.WinInst, meta).py_version
+        elif dtype == "sdist":
+            py_version = "source"
         else:
-            py_version = None
+            # This should not be reached.
+            raise ValueError
 
         return cls(
             filename, comment, cast(CheckedDistribution, meta), py_version, dtype