templater: fix reading of templates in frozen binaries with py3 < 3.7

When using a frozen binary with py3 < 3.7, there's no
`importlib.resources` module, so we use the code path that reads the
resources from the file system. That code path expects bytes for
package name and resource name.

Differential Revision: https://phab.mercurial-scm.org/D9008

mercurial/templater.py

@@ -1105,8 +1105,8 b' def open_template(name, templatepath=Non'
         return f, open(f, mode='rb')
 
     # Otherwise try to read it using the resources API
-    name_parts = pycompat.sysstr(name).split('/')
-    package_name = '.'.join(['mercurial', 'templates'] + name_parts[:-1])
+    name_parts = name.split(b'/')
+    package_name = b'.'.join([b'mercurial', b'templates'] + name_parts[:-1])
     return (
         name,
         resourceutil.open_resource(package_name, name_parts[-1]),