Fixes pypa#920. Don't replace unserscores with hyphens in distribution name.

Author: navrkald

tests/test_package.py

@@ -121,6 +121,7 @@ def test_package_signed_name_is_correct():
         (string.digits, string.digits),
         (string.punctuation, "-.-"),
         ("mosaik.SimConfig", "mosaik.SimConfig"),
+        ("package_name", "package_name"),
         ("mosaik$$$$.SimConfig", "mosaik-.SimConfig"),
     ],
 )

twine/package.py

@@ -51,12 +51,12 @@
 def _safe_name(name: str) -> str:
     """Convert an arbitrary string to a standard distribution name.
 
-    Any runs of non-alphanumeric/. characters are replaced with a single '-'.
+    Any runs of non-alphanumeric/./_ characters are replaced with a single '-'.
 
     Copied from pkg_resources.safe_name for compatibility with warehouse.
     See https://github.com/pypa/twine/issues/743.
     """
-    return re.sub("[^A-Za-z0-9.]+", "-", name)
+    return re.sub("[^A-Za-z0-9._]+", "-", name)
 
 
 class PackageFile: