Don't allow PYTHONEXECUTABLE env to be inherited by kernel process

    
it can break Python.

IPython/kernel/manager.py

@@ -228,12 +228,12 b' class KernelManager(ConnectionFileMixin):'
         # build the Popen cmd
         extra_arguments = kw.pop('extra_arguments', [])
         kernel_cmd = self.format_kernel_cmd(extra_arguments=extra_arguments)
-        if self.kernel_cmd:
-            # If kernel_cmd has been set manually, don't refer to a kernel spec
-            env = os.environ
-        else:
-            # Environment variables from kernel spec are added to os.environ
             env = os.environ.copy()
+        # Don't allow PYTHONEXECUTABLE to be passed to kernel process.
+        # If set, it can bork all the things.
+        env.pop('PYTHONEXECUTABLE', None)
+        if not self.kernel_cmd:
+            # If kernel_cmd has been set manually, don't refer to a kernel spec
             env.update(self.kernel_spec.env or {})
         # launch the kernel subprocess
         self.kernel = self._launch_kernel(kernel_cmd, env=env,