Trac #31333: sage.env.sage_include_directories: Remove hard dependency on numpy

(extracted from and needed for #29865)

`sage_include_directories` returns the basic list of include directories
for compiling extensions, not including directories obtained through
pkgconfig etc.  This basic list includes directories supplied by numpy.
This is fine for the monolithic Sage library because `numpy` is a
standard package; but in the course of modularization, we will have
various distributions that do not need the dependency on numpy.
We use try/expect around the import and call to numpy.

URL: https://trac.sagemath.org/31333
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): François Bissey

Author: Release Manager

src/sage/env.py

@@ -317,7 +317,7 @@ def _get_shared_lib_path(*libnames: str) -> Optional[str]:
 GAP_SO = var("GAP_SO", _get_shared_lib_path("gap", ""))
 
 # post process
-if ' ' in DOT_SAGE:
+if DOT_SAGE is not None and ' ' in DOT_SAGE:
     if UNAME[:6] == 'CYGWIN':
         # on windows/cygwin it is typical for the home directory
         # to have a space in it.  Fortunately, users also have
@@ -378,14 +378,18 @@ def sage_include_directories(use_sources=False):
         sage: any(os.path.isfile(os.path.join(d, file)) for d in dirs)
         True
     """
-    import numpy
     import distutils.sysconfig
 
     TOP = SAGE_SRC if use_sources else SAGE_LIB
 
-    return [TOP,
-            distutils.sysconfig.get_python_inc(),
-            numpy.get_include()]
+    dirs = [TOP,
+            distutils.sysconfig.get_python_inc()]
+    try:
+        import numpy
+        dirs.append(numpy.get_include())
+    except ModuleNotFoundError:
+        pass
+    return dirs
 
 def get_cblas_pc_module_name() -> str:
     """