Rework __main__.py to follow best practices (#1124)

It also simplifies how the `main()` function could be tested.

httpie/__main__.py

@@ -1,7 +1,6 @@
 """The main entry point. Invoke as `http' or `python -m httpie'.
 
 """
-import sys
 
 
 def main():
@@ -12,8 +11,9 @@ def main():
         from httpie.status import ExitStatus
         exit_status = ExitStatus.ERROR_CTRL_C
 
-    sys.exit(exit_status.value)
+    return exit_status.value
 
 
-if __name__ == '__main__':
-    main()
+if __name__ == '__main__':  # pragma: nocover
+    import sys
+    sys.exit(main())