guix: pyproject-build-system: Default configure-flags to empty dictionary.

PEP 517 specifies it should be a dictionary and thus meson-python cannot
handle an empty list.

Fixes: <https://issues.guix.gnu.org/62781>

* guix/build-system/pyproject.scm (lower): Change default of configure-flags.
* gnu/packages/build-tools.scm (meson-python)[arguments]: Remove #:build-backend,
which was a workaround for this bug.
[native-inputs]: Remove python-p~a-build and python-wheel, which are
not required when using the default build system.

Change-Id: I240ced5c88cf55cbee0d6005d7657bfb6b653b12

gnu/packages/build-tools.scm

@@ -344,10 +344,7 @@ (define-public meson-python
                 "0vyjhjabvm41hqijifk33idbdl62i76kfyf884f9rs29rpp77nzx"))))
     (build-system pyproject-build-system)
     (arguments
-     ;; The project is configured to use itself to build ('mesonpy') and fails;
-     ;; use another PEP 517 build system.
-     (list #:build-backend "setuptools.build_meta"
-           #:test-flags #~(list "tests"
+     (list #:test-flags #~(list "tests"
                                 ;; The test_pep518 tries to install
                                 ;; dependencies from the network using pip.
                                 "-k" "not test_pep518")
@@ -368,10 +365,7 @@ (define-public meson-python
            python-typing-extensions
            python-wheel))
     (native-inputs
-     (list python-pypa-build
-           python-wheel
-
-           ;; For tests.
+     (list ;; For tests.
            git-minimal/pinned
            patchelf
            pkg-config

guix/build-system/pyproject.scm

@@ -93,7 +93,7 @@ (define private-keywords
 (define* (pyproject-build name inputs
                           #:key source
                           (tests? #t)
-                          (configure-flags ''())
+                          (configure-flags ''(@))
                           (backend-path #f)
                           (build-backend #f)
                           (test-backend #f)