Fix twine( check) with the newly released pkginfo 1.9. (#941)

It (the pkginfo release) removes `distribution.must_decode`, though that
function was questionably public as it was an "exposed" compatibility
function (removed as part of pkginfo dropping 2.7 support).

See https://bazaar.launchpad.net/~tseaver/pkginfo/trunk/revision/205

twine/wheel.py

@@ -86,6 +86,6 @@ def read_file(name: str) -> bytes:
     def parse(self, data: bytes) -> None:
         super().parse(data)
 
-        fp = io.StringIO(distribution.must_decode(data))
+        fp = io.StringIO(data.decode("utf-8", errors="replace"))
         msg = distribution.parse(fp)
         self.description = msg.get_payload()