Feature request: Optionally use the --platform pip flag for older OS compatibility

[carlosperate]

Related to this Mu issue:

• Mu 1.2.0 fails to open in macOS 10.12 Sierra mu#2368

Basically, because it's quite uncommon to keep old macOS installations around to build Mu we could update pup to use the --platform flag with pip to ensure it downloads/installs wheels with older OS compatibility.

For example, if Mu is built in macOS 10.15 or newer, it'll likely collect/install whells for macosx_10_15_universal2, but we'd like to specify the macosx_10_12_x86_64 platform to maintain compatibility with macOS 10.12 (min version supported by PyQt5 5.13).

This might also open the possibility for building mac aarch64 installers in a x86_64 system.

[tmontes]

INVESTIGATION:

• pip can take "CLI flags" from env vars (see https://pip.pypa.io/en/stable/topics/configuration/#environment-variables)
• That being the case, maybe we're better off documenting that fact, instead of adding unnecessary complexity to the code.

NEXT:

• Try this out by setting PIP_PLATFORM before launching pup. :)