py3: have bytes version of sys.executable

sys.executable on Python 3 returns unicodes and we want bytes. So this patch
adds a new pycompat.sysexecutable which returns bytes by encoding using
os.fsencode() since it is path variable.

mercurial/pycompat.py

@@ -52,6 +52,9 b' if ispy3:'
     # returns bytes.
     getcwd = os.getcwdb
     sysplatform = sys.platform.encode('ascii')
+    sysexecutable = sys.executable
+    if sysexecutable:
+        sysexecutable = os.fsencode(sysexecutable)
 
     # TODO: .buffer might not exist if std streams were replaced; we'll need
     # a silly wrapper to make a bytes stream backed by a unicode one.
@@ -158,6 +161,7 b' else:'
     sysplatform = sys.platform
     getcwd = os.getcwd
     osgetenv = os.getenv
+    sysexecutable = sys.executable
 
 stringio = io.StringIO
 empty = _queue.Empty