tests: unconditionalize bchr

We could probably just do bytes([x]) everywhere. But this eliminates
use of PYTHON3.

Differential Revision: https://phab.mercurial-scm.org/D12238

tests/run-tests.py

@@ -1655,9 +1655,7 b' checkcodeglobpats = ['
     re.compile(br'.*\$LOCALIP.*$'),
 ]
 
-bchr = chr
-if PYTHON3:
-    bchr = lambda x: bytes([x])
+bchr = lambda x: bytes([x])
 
 WARN_UNDEFINED = 1
 WARN_YES = 2