Ensure key_from_req returns consistent canonicalized names

Fixes jazzband#1570

Remove assert:
- assert doesn't belong in non-test code
- if key's not a str, canonicalize should fail

piptools/utils.py

@@ -69,9 +69,7 @@ def key_from_req(
     else:
         # from packaging, such as install requirements from requirements.txt
         key = req.name
-    assert isinstance(key, str)
-    key = key.replace("_", "-").lower()
-    return key
+    return str(canonicalize_name(key))
 
 
 def comment(text: str) -> str: