tests: enable import checker for all python files (including no .py files)

i18n/posplit is excluded as it couldn't be trivially fixed. That's the same
as 99a2bdad0fda.

contrib/import-checker.py

@@ -663,7 +663,7 b' def sources(f, modname):'
     the input file.
     """
     py = False
-    if f.endswith('.py'):
+    if not f.endswith('.t'):
         with open(f) as src:
             yield src.read(), modname, f, 0
             py = True

tests/test-check-module-imports.t

@@ -156,12 +156,13 b' these may expose other cycles.'
 Known-bad files are excluded by -X as some of them would produce unstable
 outputs, which should be fixed later.
 
-  $ hg locate '**.py' \
+  $ hg locate 'set:**.py or grep(r"^#!.*?python")' \
   > 'tests/**.t' \
   > -X contrib/debugshell.py \
   > -X contrib/win32/hgwebdir_wsgi.py \
   > -X doc/gendoc.py \
   > -X doc/hgmanpage.py \
+  > -X i18n/posplit \
   > -X tests/test-hgweb-auth.py \
   > -X tests/hypothesishelpers.py \
   > -X tests/test-ctxmanager.py \