Move SMTP driver code into a function

pytest_localserver.smtp allows running itself as a module to start up
an SMTP server, which is implemented by a bit of driver code guarded by
an `if __name__ == "__main__"` block. Typical best practice is to put
that code in a `main()` function, which is what I'm doing in this
commit.

Author: diazona

pytest_localserver/smtp.py

@@ -156,7 +156,7 @@ def __repr__(self):  # pragma: no cover
         return "<smtp.Server %s:%s>" % self.addr
 
 
-if __name__ == "__main__":  # pragma: no cover
+def main():
     import time
 
     server = Server()
@@ -178,3 +178,7 @@ def __repr__(self):  # pragma: no cover
         # support for Python 2.4 dictates that try ... finally is not used
         # together with any except statements
         pass
+
+
+if __name__ == "__main__":  # pragma: no cover
+    main()