iotests: touch up log function signature

iotests: touch up log function signature

Representing nested, recursive data structures in mypy is notoriously
difficult; the best we can reliably do right now is denote the leaf
types as "Any" while describing the general shape of the data.

Regardless, this fully annotates the log() function.

Typing notes:

TypeVar is a Type variable that can optionally be constrained by a
sequence of possible types. This variable is bound to a specific type
per-invocation, like a Generic.

log() behaves as log<Msg>() now, where the incoming type informs the
signature it expects for any filter arguments passed in. If Msg is a
str, then filter should take and return a str.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200331000014.11581-9-jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>