Add capability for python-venv to use --upgrade

Fixes #27

scone/default/recipes/python.py

@@ -51,6 +51,8 @@ class PythonVenv(Recipe):
 
         self.no_apt_install = check_type(args.get("_no_apt_install", False), bool)
 
+        self.upgrade = check_type(args.get("upgrade", False), bool)
+
         # TODO(sdists)
 
     def prepare(self, preparation: Preparation, head: Head):
@@ -80,6 +82,10 @@ class PythonVenv(Recipe):
         await exec_no_fails(kitchen, [self.interpreter, "-m", "venv", self.dir], "/")
 
         install_args = []
+
+        if self.upgrade:
+            install_args.append("--upgrade")
+
         for name, flags in self.install:
             if "-r" in flags:
                 install_args.append("-r")