Specifying the particular directory with tests
significantly speeds up their collection and execution.

Before:

```console
$ time pytest -k nothing
12 deselected in 1.36s
```

After:

```
12 deselected in 0.04s
```

1. Review `Pipfile`:
  - Remove unused `filelock`;
  - Install `mypy` - static type checker for Python.
2. Add new command `pipenv run lint`.
3. Enable `mypy` in CI. Fix reported errors in `test_basics.py`.
4. Renew readme.