Fixes the build.
Changes proposed in this pull request:
- apply all of the isort changes that resulted from the latest version
- pin to the latest version (4.3.15) so this doesn't happen again, unless we bump the version on purpose
- Added a note to create aiohttp handlers;
- Added a new parameter on AioHttpApp.__init__ to choose create only one api. With this parameter we can create an api with a empty base_path;
- Added exceptions messages on AioHttpApp.add_api method;
- Created new tests for AioHttpApp.add_api.