hghave: add "py3k" feature to check whether test runs with Python 3.x

This patch adds "py3k" feature to check whether test runs with Python
3.x.

This check is needed for portability of test code: for example, in the
default, modules are imported relatively first with python 2.x, but
imported absolutely with Python 3.x.

tests/hghave.py

@@ -282,6 +282,9 b' def has_absimport():'
     from mercurial import util
     return util.safehasattr(__future__, "absolute_import")
 
+def has_py3k():
+    return 3 == sys.version_info[0]
+
 checks = {
     "true": (lambda: True, "yak shaving"),
     "false": (lambda: False, "nail clipper"),
@@ -324,4 +327,5 b' checks = {'
     "msys": (has_msys, "Windows with MSYS"),
     "aix": (has_aix, "AIX"),
     "absimport": (has_absimport, "absolute_import in __future__"),
+    "py3k": (has_py3k, "running with Python 3.x"),
 }