Minor update to audio/pithos

[jhibbits]

Summary

Two updates for the price of one:

• audio/pithos does not need devel/git for running, so remove this dependency which conflicts with other versions of git (devel/git-subversion)
• pithos really requires gstreamer1, so make it a direct dependency rather than an indirect dependency through py36-gstreamer1

Test Plan

Build and run audio/pithos

[jhixson]

Since this has been committed already, closing this out.