py3: unbyteify arguments to warnings.filterwarnings() stable

py3: unbyteify arguments to warnings.filterwarnings()

This fixes a crash when trying to import the convert extension on Python 3.

--- a/hgext/convert/subversion.py	Sat Jun 06 19:15:11 2020 +0800
+++ b/hgext/convert/subversion.py	Mon Jun 15 03:09:55 2020 +0200
@@ -55,7 +55,7 @@
     import warnings
 
     warnings.filterwarnings(
-        b'ignore', module=b'svn.core', category=DeprecationWarning
+        'ignore', module='svn.core', category=DeprecationWarning
     )
     svn.core.SubversionException  # trigger import to catch error