Fix type errors by explicitly exporting vobject.readComponents?

[dbarnett]

I found type checking my code that uses vobject gave me an error for trying to use vobject.readComponents:

gcalcli/ics.py:24: error: Module "vobject" does not explicitly export attribute "readComponents"  [attr-defined]

Typeshed does supply types for the vobject module, but those don't declare readComponents at all. I think if you can make a trivial change here to explicitly export it then I can send a trivial PR to typeshed to regenerate using the new version.

The relevant mypy docs for "no-implicit-reexport" suggest one of these two changes to explicitly export:

1. Add explicit alias as readComponents on the import
2. Define __all__ = ['readComponents'...] on the file

WDYT?