Crashtest

Fixed

- Fixed too narrow type in `Inspector` class

Fixed
- Fixed a decoding error by forcing 'utf-8' encoding when opening a file. (5)

Misc
- Added typing
- Fixed multiple issues raised by `flake8` and plugins
- Updated dependencies (both package and dev)
- Bumped Python versions to 3.7+

Fixed

- Fixed an error when trying to retrieve the line of a frame with no context ([2](https://github.com/sdispater/crashtest/pull/2)).