utils/pypi: use python from formula for non-pypi url metadata

[cho-m]

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This fixes the resource detection when formula has Python packages that are not compatible with current aliased python formula, e.g. awscli

Also switch to opt_libexec path to ignore Python formula patch version and revision bump differences.

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Originally hit when working on Homebrew/homebrew-core#163705, e.g.

❯ brew update-python-resources awscli
==> Retrieving PyPI dependencies for "https://github.com/aws/aws-cli/archive/refs/tags/2.15.22.tar.gz
==> Retrieving PyPI dependencies for excluded ""...
Error: Unable to determine metadata for "https://github.com/aws/aws-cli/archive/refs/tags/2.15.22.tar.gz" because of a failure when running
`/opt/homebrew/opt/python@3.12/bin/python3 -m pip install -q --no-deps --dry-run --ignore-installed --report /dev/stdout https://github.com/aws/aws-cli/archive/refs/tags/2.15.22.tar.gz`.
Please report this issue:
  https://docs.brew.sh/Troubleshooting
Warning: Removed Sorbet lines from backtrace!
Rerun with `--verbose` to see the original backtrace
/opt/homebrew/Library/Homebrew/utils/pypi.rb:147:in `basic_metadata'
/opt/homebrew/Library/Homebrew/utils/pypi.rb:26:in `name'
/opt/homebrew/Library/Homebrew/utils/pypi.rb:304:in `update_python_resources!'
/opt/homebrew/Library/Homebrew/dev-cmd/update-python-resources.rb:41:in `block in update_python_resources'
/opt/homebrew/Library/Homebrew/dev-cmd/update-python-resources.rb:40:in `each'
/opt/homebrew/Library/Homebrew/dev-cmd/update-python-resources.rb:40:in `update_python_resources'
/opt/homebrew/Library/Homebrew/brew.rb:86:in `<main>'

            _ruamel_yaml.c:27082:36: error: no member named 'curexc_traceback' in 'struct _ts'
                    PyObject* tmp_tb = tstate->curexc_traceback;
                                       ~~~~~~  ^
            _ruamel_yaml.c:27085:21: error: no member named 'curexc_traceback' in 'struct _ts'
                        tstate->curexc_traceback = tb;
                        ~~~~~~  ^
            _ruamel_yaml.c:27624:22: error: call to undeclared function '_PyUnicode_Ready'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                    if (unlikely(__Pyx_PyUnicode_READY(s1) < 0) || unlikely(__Pyx_PyUnicode_READY(s2) < 0))
                                 ^

[MikeMcQuaid]

Looks great, thanks @cho-m!