More pip package and supports 3.0.1 release

[smellman]

I added following:

• use matplotlib and gdal from pip package instead of homebrew packages
• use Homebrew's gdal
• use 3.0.1 release
• bugfix: set python3 for python_exec
• WIP: venv support

I suggest to use Homebrew's gdal (currently gdal is 2.2.4!) instead of osgeo4mac's gdal.
But it mean drop grass7 support because it depends osgeo4mac's gdal.
Currently, brew install --no-sandbox qgis3-dev raise error in Finishing up task but QGIS.app can run.

==> Finishing up
Error: Failed changing dylib ID of /usr/local/Cellar/qgis3-dev/3.0.1/libexec/python/lib/python/site-packages/matplotlib/.dylibs/libpng16.16.dylib
  from /DLC/matplotlib/libpng16.16.dylib
    to /usr/local/opt/qgis3-dev/libexec/python/lib/python/site-packages/matplotlib/.dylibs/libpng16.16.dylib
Error: Failed to fix install linkage
The formula built, but you may encounter issues using it or linking other
formula against it.

Also, I try venv but it fail when running setup.py in matplotlib from tar ball.
Someone try this fixes?

[3nids]

I opened a PR in main Osgeo repo for QGIS 3 and integrated your modifications.
OSGeo/homebrew-osgeo4mac#353

[smellman]

@3nids Thanks! and now it supports venv!