This reverts commit 85271644b0083cbc0fd6fea71120d1ad859fbc2a.
Alex noticed that setting PYTHONPATH causes weirdness if we depend_on
something which may be optionally built --with-python3; PYTHONPATH
unexpectedly contains python3 modules in the depending formula if
the formula upon which it depends was built --with-python3 even
though the depending formula may only use python2.
ClosesHomebrew/homebrew#43724. ClosesHomebrew/homebrew#43744.
Dependency is another similar, related class and it's super confusing
to have some Requirements that are named *Dependency.
ClosesHomebrew/homebrew#38891.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>