[FIX] P3: repair parsing of lessc compilation error

Coercing the result of the compilation to text is easier and more
portable than P3-specific code for putting the Popen streams in text
mode.

Fixes #19659

odoo/addons/base/ir/ir_qweb/assetsbundle.py

@@ -400,7 +400,7 @@ class AssetsBundle(object):
             return ''
         result = compiler.communicate(input=source.encode('utf-8'))
         if compiler.returncode:
-            cmd_output = ''.join(result)
+            cmd_output = ''.join(misc.ustr(result))
             if not cmd_output:
                 cmd_output = "Process exited with return code %d\n" % compiler.returncode
             error = self.get_preprocessor_error(cmd_output, source=source)