execnet testsuite failures on Windows (PyPy)

[nicoddemus]

The following tests are xfailing on Windows in PyPy:

test_remote_exec_function_with_kwargs[sys.executable]
test_remote_exec_inspect_stack
TestMakegateway.test_popen_default
TestMakegateway.test_popen_explicit
TestMakegateway.test_popen_chdir_absolute
TestMakegateway.test_popen_chdir_newsub
test_popen_stderr_tracing
test_termination_on_remote_channel_receive

There's more details on #62