[PyROOT] Skip numba test also if numba is not available

So far, the test was only skipped if the numba version was too low, but
it might also happen that numba is not installed at all. For example if
one uses Python 3.13, which doesn't support numba yet.

python/numba/PyROOT_numbatests.py

@@ -1,17 +1,25 @@
-import sys
+import importlib
 import os
 import pytest
+import sys
 import time
 
 import ROOT
-import numba
 
 sys.path.append(os.path.dirname(os.path.dirname(__file__)))
 
+def has_required_numba():
+    """Check if numba is available and that the version matches requirement."""
+    if not importlib.util.find_spec("numba"):
+        return False
+    import numba
+
+    # With fallback in case it's an older numba version that doesn't have the
+    # version_info attribute yet:
+    return getattr(numba, "version_info", (0, 0)) >= (0, 54)
 
-@pytest.mark.skipif(
-        not hasattr(numba, 'version_info') or numba.version_info < (0, 54),
-        reason="Numba version 0.54 or more required")
+
+@pytest.mark.skipif(not has_required_numba(), reason="Numba version >=0.54 required")
 class TestClasNumba:
     """Tests numba support for PyROOT"""
 
@@ -35,6 +43,7 @@ def test01_simple_free_func(self):
         import ROOT.NumbaExt
         import math
         import numpy as np
+        import numba
 
         def go_slow(a):
             trace = 0.0
@@ -57,6 +66,7 @@ def go_fast(a):
     def test02_member_function(self):
         import ROOT.NumbaExt
         import math
+        import numba
 
         # Obtain a vector of ROOT::Math::LorentzVector from the sample
         # .root file
@@ -78,8 +88,9 @@ def numba_calc_pt_vec(vec_lv):
 
     def test03_inheritance(self):
         """This test shows one of the limitations of the current support"""
-
+        import numba
         from numba.core.errors import TypingError
+
         errtyp = TypingError if numba.version_info < (0, 60) else KeyError
 
         ROOT.gInterpreter.Declare("""