Build Python wheels

[arahlin]

This PR rearranges this code to facilitate building and installing the spt3g python package with standard python tools like pip and build.

The shared libraries, headers, and cmake configuration files are installed relative to CMAKE_INSTALL_PREFIX as usual. The pip installer handles versioning, and passes appropriate cmake definitions to the cmake builder that compiles all of the libraries and modules, then installs the python package into the appropriate location, and triggers the cmake installer.

The pip installer can also be run in an --editable mode that works much like the cmake build directory, enabling code development without the need to make changes to various path environment variables. It also should install all of the components into appropriate locations when run in a virtual environment.

Versioning is done using setuptools_scm with the pip build system, or with a custom shell script with the cmake build system. In both cases, the version information is determined by running a series of git commands or by parsing the .git_archival.txt file, then output to spt3g/version.py for access in the Python package and to the SPT3G_VERSION preprocessor directive for access in C++.

Finally, this PR includes a github workflow for producing pre-compiled wheels for Linux and OSX platforms.

[tskisner]

These changes look good. Give me a little time to try running a test within the so3g wheel builds and see if this branch removes the need for our local patching.

[arahlin]

Ok I've rearranged a bit so that the bits that are specific to the GitHub runner just check the CIBUILDHWEEL environment variable, and otherwise the python build assembles everything that the cmake one did. I've also tried to make the cmake version handling a bit smoother.

[arahlin]

The runner now handles linux and macOS wheels identically, not building any executables that require linking against libpython, and linking all shared libraries and modules against the Python::Module smart target with dynamic lookups enabled. The parts of the build specific to cibuildwheel are otherwise very few and well-contained.

[arahlin]

I've split off the library rearranging bits into a separate PR #163 to make this one a bit less of a monster.