Run pip without the --build flag

Previously, fpm would use `pip download ... --build ...` to instruct pip
to unpack a given python package to a specific directory for the purpose
of running something like `python setup.py` from it.

However, somewhere in 2021, pip removed this flag. First, I think, it
was deprecated and ignored, then finally removed. One reference to
this removal in the upstream pip project is this issue:
https://github.com/pypa/pip/issues/8333

Without `--build`, pip will place a single tarball in the destination
directory. Fpm cannot easily predict the name of this file because we
don't know the "real" name of the python package nor do we know the
version number being downloaded.

For example:

```
% python3 -m pip download --no-binary :all: --no-deps --no-clean django
...
Successfully downloaded django
% ls
Django-4.0.4.tar.gz
```

Best guess:
* we can expect exactly one file in the previously-empty target directory
* we can also expect that it is a .tar.gz

I don't know if these guesses are always correct, but it's a start.

As of this commit, the following command generates a Debian package:

`fpm -s python --python-bin python3 -t deb django`

Prior to this commit, with a newer version of pip, the command would
fail.

Fixes #1831

lib/fpm/package/python.rb

@@ -162,13 +162,22 @@ class FPM::Package::Python < FPM::Package
         ]
       end
 
-      setup_cmd += [
-        "--build",
-        target,
-        want_pkg,
-      ]
-      
+      setup_cmd << want_pkg
+
       safesystem(*setup_cmd)
+
+      # Pip removed the --build flag sometime in 2021, it seems: https://github.com/pypa/pip/issues/8333
+      # A workaround for pip removing the `--build` flag. Previously, `pip download --build ...` would leave
+      # behind a directory with the Python package extracted and ready to be used.
+      # For example, `pip download ... Django` puts `Django-4.0.4.tar.tz` into the build_path directory.
+      # If we expect `pip` to leave an unknown-named file in the `build_path` directory, let's check for
+      # a single file and unpack it.  I don't know if it will /always/ be a .tar.gz though.
+      files = ::Dir.glob(File.join(build_path, "*.tar.gz"))
+      if files.length != 1
+        raise "Unexpected directory layout after `pip download ...`. This might be an fpm bug? The directory is #{build_path}"
+      end
+
+      safesystem("tar", "-zxf", files[0], "-C", target)
     else
       # no pip, use easy_install
       logger.debug("no pip, defaulting to easy_install", :easy_install => attributes[:python_easyinstall])