feat(rez-pip): add pip version check to ensure pep517 compatibility
feat(rez-pip): add pip version check to ensure pep517 compatibility

rez will now enforce wheel distribution based installations. A version
of pip >= 19 is required in order to do so. Add pip version check
to the callbacks of rez-pip command to validate that an up to date
pip is being used.

Relates: https://github.com/nerdvegas/rez/pull/602 https://github.com/nerdvegas/rez/pull/628 https://pip.pypa.io/en/stable/news/#id49

instinct-vfx/rez,instinct-vfx/rez,nerdvegas/rez,nerdvegas/rez