python-dateutil will be updated to version 2.x and there may be changes that affect this package. If python-matplotlib will work with the newer python-dateutil, you can safely close this bug. If python-matplotlib really does require python-dateutil 1.5, please use Requires: python-dateutil15 for Fedora versions of the package until it can be updated.
https://github.com/matplotlib/matplotlib/blob/cfb6ac64578c1d9e042a55c1c62894ba0d9bf331/setupext.py#L1020 talks about dateutil 2.1 and 2.0, so things should be fine. dateutil is required for dates on axis. Should be easy to test.
I now tested and things seem to work with both matplotlib from F21 as from the upcoming matplotlib 1.4.2.