Minor clean-up (#1078)

- Remove default arguments to `open()`.
- Make use of `pytest` mechanisms for temporary folders.

httpie/plugins/__init__.py

@@ -7,3 +7,5 @@ from .base import (
     AuthPlugin, FormatterPlugin,
     ConverterPlugin, TransportPlugin
 )
+
+__all__ = ('AuthPlugin', 'ConverterPlugin', 'FormatterPlugin', 'TransportPlugin')