setup: install pip in virtualenv to make sure we have the latest version

Older versions are good enough for bootstrapping (and might also be good for
everything else) but gives:

  WARNING: You are using pip version 19.3.1; however, version 20.0.2 is available.
  You should consider upgrading via the 'pip install --upgrade pip' command.

The user might still get this warning initially, but then it goes away ...

setup.py

@@ -72,6 +72,7 b' requirements = ['
     "paginate >= 0.5, < 0.6",
     "paginate_sqlalchemy >= 0.3.0, < 0.4",
     "bcrypt >= 3.1.0, < 3.2",
+    "pip >= 20.0, < 999",
 ]
 
 dependency_links = [