v1.2.0

• adding support for darwin platform
• fetching metadata from pypi using aiohttp, #42
• for sources use sha256 instead of md5. when sha256 is not provided we download package and calculate it, #41
• python version option (-V) is now a required option, #39
• python "object" now has a pkgs_top_level list of all top level packages, #38
• an option -T added to turn doCheck=true; for all generated packages, #36
• detect more licenses, #29
• multiple requirements.txt can now be provided when calling pypi2nix, #34
• run nix-shell with --pure option