FIX: Force exit in equilibrium solidification after a successful binary search

[bocklund]

• Exit the equilibrium solidification while loop after a binary search, because the solidification must be converged then by definition (fixes a floating point precision issue where 0.999... < 1 caused the loop to be infinite on some platforms.
• Update CI for pip version of pycalphad
• Remove environment.yml development file
• Drop Python 3.6