Tweak tox.ini to get it working well again

setup.py

@@ -5,15 +5,18 @@
 import sgp4
 description, long_description = sgp4.__doc__.split('\n', 1)
 
-# Force compilation on Travis CI + Python 3 to make sure it keeps working.
-optional = True
-if sys.version_info[0] != 2 and os.environ.get('TRAVIS') == 'true':
-    optional = False
-
 # It is hard to write C extensions that support both Python 2 and 3, so
 # we opt here to support the acceleration only for Python 3.
+
 ext_modules = []
 if sys.version_info[0] == 3:
+
+    # This lets CI force us to exit with an error if compilation fails,
+    # instead of falling back silently to the backup Python code.
+    optional = True
+    if os.environ.get('SGP4_FORCE_COMPILE') == 'true':
+        optional = False
+
     ext_modules.append(Extension(
         'sgp4.vallado_cpp',
         optional=optional,

tox.ini

@@ -1,19 +1,14 @@
 [tox]
-envlist = py27, py38
+envlist = py27, py312
 
 [testenv]
 changedir = {envtmpdir}
 deps =
     numpy
+whitelist_externals = python
 commands = python -m unittest discover -b {posargs} sgp4
 
 # Force the C++ to compile under Python 3, so that we really test the
 # extension and not accidentally test the pure Python code over again
-# (which is already tested under Python 2 in the env below).
-setenv = TRAVIS = true
-
-[testenv:py26]
-deps =
-    numpy
-    unittest2
-commands = python -m {envbindir}/unit2 discover {posargs} sgp4
+# (which is already tested under Python 2).
+setenv = SGP4_FORCE_COMPILE = true