Update formulas to use latest Python incantations

These formulas now use the python object in the Formula class to obtain
the local and global site_packages paths and the Python executable.
This removes the need for the which_python utility function and also
improves support for various versions of System / Homebrew Python
(see Homebrew PR 18588 for more details of the python object).
owlcat: Fix revision to r8761

It is necessary to fix owlcat to r8761 as the next revision contains
both a Pyxis directory and pyxis script in the root directory, which
clash on a case-insensitive HFS+ filesystem. This is very temporary.
Update formulas to use latest fortran incantations

The use of ENV.fortran has been replaced by depends_on :fortran.
See Homebrew PR 13621 for more details.