boost Python variants should be subports for automated installs with CI and buildbots

[essandess (Steve Smith)]

The Python version variants of boost are implemented as variants, not subports. This breaks CI and buildbot builds pf ports that depend on boost being installed with a specific Python version, e.g., py-graph-tool, ​https://github.com/macports/macports-ports/pull/17914#issuecomment-1474822953

Additionally, the correct way to implement such dependencies is via subports, comment:ticket:126:48:

"The progress that has been made is that MacPorts now supports subports, which make it easier for a single portfile to provide multiple separately-installable ports. Ports that provide optional functionality that other ports might need to depend on should break that optional functionality out into subports instead of using variants. We should not pursue the idea of allowing variants to be depended upon."

These variants should be converted to subports like boost176-python311 etc., which could have the added benefit of avoiding current conflicts between these python port variants.