gh-39997: Run python3 directly instead of `sage -python` in sage/tests/startup.py
    
`sage -python` is no longer available in the meson build
    
URL: #39997
Reported by: Antonio Rojas
Reviewer(s): Antonio Rojas, Tobias Diez

Author: Release Manager

src/sage/tests/startup.py

@@ -8,7 +8,7 @@
     sage: from sage.tests.cmdline import test_executable
     sage: environment = "sage.all"
     sage: cmd = f"from {environment} import *\nprint('IPython' in sys.modules)\n"
-    sage: print(test_executable(["sage", "--python"], cmd)[0])  # long time
+    sage: print(test_executable(["python3"], cmd)[0])  # long time
     False
 
 Check that numpy (:issue:`11714`) and pyparsing are not imported on startup