Implement async def, async for, and await syntax for method extraction

[lieryan]

Fix #375

Todo:

• fix version specifier
• Implement async def, async for, and await syntax for method extraction #389 (comment)
• Implement async def, async for, and await syntax for method extraction #389 (comment)

[lieryan]

Currently still failing cases:

    @testutils.only_for_versions_higher('3.5')
    def test_extract_async_for_loop_2(self):
        code = dedent('''\
            async def my_func(my_list):
                async for x in my_list:
                    var = x + 1
                return var
        ''')
        start, end = self._convert_line_range_to_offset(code, 2, 3)
        refactored = self.do_extract_method(code, start, end, 'new_func')
        expected = dedent('''\
            async def my_func(my_list):
                var = new_func()
                return var

            def new_func():
                async for x in my_list:
                    var = x + 1
                return var
        ''')
        self.assertEqual(expected, refactored)

[lieryan]

Currently still failing cases:

    @testutils.only_for_versions_higher('3.5')
    def test_extract_inner_async_function(self):
        code = dedent('''\
            def my_func(my_list):
                async def inner_func(my_list):
                    for x in my_list:
                        var = x + 1
                return inner_func
        ''')
        start, end = self._convert_line_range_to_offset(code, 2, 4)
        refactored = self.do_extract_method(code, start, end, 'new_func')
        expected = dedent('''\
            def my_func(my_list):
                inner_func = new_func(my_list)
                return new_func

            def new_func(my_list):
                async def inner_func(my_list):
                    for x in my_list:
                        var = x + 1
                return inner_func
        ''')
        self.assertEqual(expected, refactored)