makepanda: test_wheel.py should upgrade pip to latest version

[skip ci]

makepanda/test_wheel.py

@@ -23,11 +23,16 @@ def test_wheel(wheel, verbose=False):
     else:
         subprocess.call([sys.executable, "-m", "virtualenv", "--clear", envdir])
 
-    # Install pytest into the environment, as well as our wheel.
+    # Make sure pip is up-to-date first.
     if sys.platform == "win32":
         pip = os.path.join(envdir, "Scripts", "pip.exe")
     else:
         pip = os.path.join(envdir, "bin", "pip")
+    if subprocess.call([pip, "install", "-U", "pip"]) != 0:
+        shutil.rmtree(envdir)
+        sys.exit(1)
+
+    # Install pytest into the environment, as well as our wheel.
     if subprocess.call([pip, "install", "pytest", wheel]) != 0:
         shutil.rmtree(envdir)
         sys.exit(1)