dev env: add typed-ast to fix mypy on python3.7

```
$ pipenv run mypy "$(cat *.egg-info/top_level.txt)" tests
The typed_ast package is not installed.
You can install it with `python3 -m pip install typed-ast`.
Error: Process completed with exit code 1.
```

fphammerle/ical2vdir@5e39467

Pipfile

@@ -14,6 +14,9 @@ pylint-import-requirements = "*"
 pytest = "*"
 pytest-cov = "*"
 
+# mypy on python<3.8
+typed-ast = {markers = "python_version < '3.8'"}
+
 [requires]
 python_version = "3"