Use "source" as python-version for sdist uploads

Although not setting the "pyversion" field in the upload data works,
and despite it not being used for much, this is what PyPI (and most
likely other indexes) expects.

This will simplify follow-up patches.

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.

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