29056: make numpy find openblas from Homebrew

For numpy to find Homebrew's openblas on macOS, this ticket adds an
`[openblas]` section to numpy's site.cfg if Sage is using openblas.

This is also what Homebrew's
[https://github.com/Homebrew/homebrew-core/blob/f2463cbba086689c872cddd3a8bc242976a603ff/Formula/numpy.rb#L25-L30
numpy formula] does.

See also the upstream
[https://github.com/numpy/numpy/blob/2199ba7fb7f8bb383552fdcd6feec4faee7ae394/site.cfg.example#L129-L132
site.cfg.example].

build/pkgs/numpy/lapack_conf.py

@@ -16,18 +16,30 @@
 pc_blas   = pkgconfig.parse('cblas blas')
 pc_lapack = pkgconfig.parse('lapack')
 
-if not (os.environ['UNAME'] == 'Darwin'):
+if os.environ['UNAME'] == 'Darwin':
+    # on macOS, if openblas is available (the default), use it instead of the
+    # macOS Accelerate framework
+    if 'openblas' in pc_blas.get('libraries', []):
+        conf_file.write('[openblas]\n')
+        conf_file.write('libraries    = '+', '.join(pc_blas['libraries'])+'\n')
+        inc_dir = pc_blas['include_dirs']
+        if len(inc_dir) > 0:
+            conf_file.write('include_dirs = '+ ':'.join(inc_dir)+'\n')
+        lib_dir = pc_blas['library_dirs']
+        if len(lib_dir) > 0:
+            conf_file.write('library_dirs = '+ ':'.join(lib_dir)+'\n')
+else:
     conf_file.write('[blas]\n')
     inc_dir = pc_blas['include_dirs']
-    if len(inc_dir) > 0 :
+    if len(inc_dir) > 0:
         conf_file.write('include_dirs = '+ ':'.join(inc_dir)+'\n')
     lib_dir = pc_blas['library_dirs']
-    if len(lib_dir) > 0 :
+    if len(lib_dir) > 0:
         conf_file.write('library_dirs = '+ ':'.join(lib_dir)+'\n')
     conf_file.write('blas_libs    = '+', '.join(pc_blas['libraries'])+'\n')
     conf_file.write('[lapack]\n')
     lib_dir = pc_lapack['library_dirs']
-    if len(lib_dir) > 0 :
+    if len(lib_dir) > 0:
         conf_file.write('library_dirs = '+ ':'.join(lib_dir)+'\n')
     conf_file.write('lapack_libs  = '+', '.join(pc_lapack['libraries'])+'\n')