Use sys.executable when testing with -m fabric

The current test case assumes /usr/bin/python is the current running
Python release, which it may not always be, so make use of
sys.executable to re-exec the interpreter.

tests/main.py

@@ -358,5 +358,5 @@ def python_dash_m_acts_like_fab(self, capsys):
 Paramiko .+
 Invoke .+
 """.strip()
-        output = run("python -m fabric --version", hide=True, in_stream=False)
+        output = run(sys.executable + " -m fabric --version", hide=True, in_stream=False)
         assert re.match(expected_output, output.stdout)