Can't build Boost Python 3 on macOS

Made a report https://github.com/boostorg/python/issues/401 in the `boostorg/boost` repo so I am transfering here to be more relevant.

Using official Python at [python.org](https://python.org), building Boost Python 3 (Boost 1.80.0) with
```sh
./bootstrap.sh --with-libraries=python --with-python=python3
./b2 --debug-configuration --debug-building --debug-generator install
```
does not work: `bootstrap.sh` seems to work as it detected the right Python root location
```
Detecting Python version... 3.10
Detecting Python root... /Library/Frameworks/Python.framework/Versions/3.10
```
However, `b2` then reports
```
notice: [python-cfg] Configuring python...
notice: [python-cfg] user-specified version: "3.10"
notice: [python-cfg] user-specified cmd-or-prefix: "/Library/Frameworks/Python.framework/Versions/3.10"
notice: [python-cfg] Checking interpreter command "/Library/Frameworks/Python.framework/Versions/3.10/bin/python"...
notice: [python-cfg] running command '/Library/Frameworks/Python.framework/Versions/3.10/bin/python -c "from sys import *; print('version=%d.%d\nplatform=%s\nprefix=%s\nexec_prefix=%s\nexecutable=%s' % (version_info[0],version_info[1],platform,prefix,exec_prefix,executable))" 2>&1'
notice: [python-cfg] ...does not invoke a working interpreter
```

Apparently, the Python 3 installer on macOS does not created `bin/python`, only `bin/python3`. Shouldn't it take into account the suggested name for the Python executable in `bootstrap` instead of ignoring it? (It builds perfectly for Python 2.7 included in macOS.)

Start by reproducing the reported commands, ./bootstrap.sh --with-libraries=python --with-python=python3 and ./b2 --debug-configuration --debug-building --debug-generator install, on macOS with the official Python 3 installer. Inspect how bootstrap.sh passes the Python root to b2 and why b2 checks bin/python rather than the available bin/python3. Done means Boost Python 3 builds successfully with this installation.

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