Make venv for target Python in tests

When possible, use venv to create the target Python in the tests.
This creates an isolated Python, and quite importantly maintains
isolation from system packages when the initial Python is itself
from a virtual environment.

slapos/rebootstrap/tests.py

@@ -50,15 +50,19 @@ class Pyinstall:
 
   def __init__(self, buildout, name, options):
     self.options = options
-    options['executable'] = os.path.join(buildout['buildout'][
-      'parts-directory'], name, 'bin', 'python')
+    self.part_dir = os.path.join(buildout['buildout']['parts-directory'], name)
+    options['executable'] = os.path.join(self.part_dir, 'bin', 'python')
 
   def install(self):
     python = self.options['executable']
     if not os.path.exists(python):
-      d = os.path.dirname(python)
-      os.path.exists(d) or os.makedirs(d)
-      shutil.copy(sys.executable, python)
+      try:
+        from venv import create
+        create(self.part_dir, clear=True)
+      except ImportError:
+        d = os.path.dirname(python)
+        os.path.exists(d) or os.makedirs(d)
+        shutil.copy(sys.executable, python)
     return []
 
   update = install