To change license (or grant access under an other license) copyright holders should agree.
This code was mostly written pre-construction and the main copyright is with LSSTC. LSSTC have signed over all copyrights to AURA (Agreement-221).
UW and Princeton also hold copyright on this code.
With this RFC I am checking there is no objection, especially from UW and Princeton, to change the license on this one package.
TimJ has already done the work to make sure this has no further impact on the codebase.
Motivation: Gemini would like to use a fork of LSST's "pex_config" module for controlling pipeline parameters in the upcoming release of Gemini's DRAGONS data reduction packages (https://github.com/GeminiDRSoftware/DRAGONS).
However DRAGONS packages are BSD licensed, as currently approved by AURA, which is not compatible with GPL.