CI/BLD: move CIBW commands to pyproject.toml, move scripts

[mwtoews]

This PR does the following:

• Move a few cibuildwheel commands from the CI workflow to pyproject.toml
• Remove unneeded vcpython27 package installation for Windows (xref this comment)
• Move a few scripts from ci/ to /scripts
• The install_libspatialindex.bash script is really just a POSIX shell (i.e. no bash features), so rename and use sh

[adamjstewart]

For fun, I tried running this script with dash on macOS. I confirmed that it's POSIX compliant. The only issue I encountered is that sha256sum does not exist on macOS, you have to use shasum -a 256 instead. Might be able to check uname in an if-statement if you want to fix this. I can also submit a PR after this one is merged if you want.

[mwtoews]

Thanks for checking! It looks like the macos image has sha256sum, probably via brew's coreutils package (the internet tells me at least). But I won't mind if you want to complete this aspect. You can push commits on this PR, if you know how.

[adamjstewart]

I confirmed that it's POSIX compliant.

I take this back. == is not POSIX compliant, we should be using = instead. I'll fix this in my commit.

[adamjstewart]

Btw, https://www.shellcheck.net/ is handy if you want to check for POSIX compliance and other sh gotchas.

[mwtoews]

Handy resource! The cli version does nice work too.