build/pkgs/pythran/install-requires.txt: add lower bound

Require at least pythran-0.12.1 for the bugfix in upstream commit
cd68ac68, "Only use numpy.distutils conditionally." Earlier versions
of pythran can emit deprecation warnings that cause the test suite to
fail; for example,

  sage: cython("cdef class A:\n    cdef public int a")
  DeprecationWarning:
      `numpy.distutils` is deprecated since NumPy 1.23.0, as a result
      of the deprecation of `distutils` itself. It will be removed for
      Python >= 3.12. For older Python versions it will remain present.
      It is recommended to use `setuptools < 60.0` for those Python versions.
      For more details, see:
        https://numpy.org/devdocs/reference/distutils_status_migration.html

build/pkgs/pythran/install-requires.txt

@@ -1 +1 @@
-pythran
+pythran >=0.12.1