build/pkgs/sagelib/src/tox.ini: Add environment python-norequirements

build/pkgs/sagelib/src/tox.ini

@@ -2,18 +2,35 @@
 #
 #   ./sage -i tox
 #
-# To build and test in the tox environment:
+# All tests require an installation of the non-Python components of the Sage distribution
+# in SAGE_LOCAL.
 #
-# ./sage -sh -c '(cd build/pkgs/sagelib/src && tox -v -v -v)'
+# To build and test in the tox environment using the concrete Python dependencies specified
+# by requirements.txt:
 #
-# To test interactively:
+#   ./sage -sh -c '(cd build/pkgs/sagelib/src && tox -v -v -v)'
 #
-# build/pkgs/sagelib/src/.tox/python/bin/python
+# Afterwards, to test interactively:
+#
+#   build/pkgs/sagelib/src/.tox/python/bin/python
+#   build/pkgs/sagelib/src/.tox/python/bin/sage
+#
+# To build and test without using the concrete dependencies specified by requirements.txt,
+# using the dependencies declared in setup.cfg only:
+#
+#   ./sage -sh -c '(cd build/pkgs/sagelib/src && tox -v -v -v -e python-norequirements)'
 #
 [tox]
 
 [testenv]
-deps = -rrequirements.txt
+deps =
+    !norequirements: -rrequirements.txt
+
+    # These are dependencies of the build system (setup.py).
+    # Later we will declare them using pyproject.toml
+    norequirements:  Cython
+    norequirements:  pkgconfig
+    norequirements:  numpy
 
 passenv =
     SAGE_LOCAL