Install wheel and cython on fbcode_builder venv environments

Summary:
This is an update to the fbcode_builder codebase to allow setting up the python virtualenv with python dependencies installed. I've included wheel and cython (with a pinned version to 0.28.6 which is the only version that works with thriftpy3 at the moment, due to https://github.com/cython/cython/issues/2985) as standard packages since these are required by some of our top-level dependencies (folly and thrift)

As far as I know, there are no other projects that use PYTHON_VENV at the moment except LogDevice so the impact should be minimal.

Reviewed By: lucaspmelo

Differential Revision: D18758383

fbshipit-source-id: 264941311c5e3a19dc4ef2bb78c9a1baa34dfd8c

build/fbcode_builder/docker_builder.py

@@ -47,10 +47,12 @@ class DockerFBCodeBuilder(FBCodeBuilder):
             ShellQuoted('FROM {}'.format(self.option('os_image'))),
             # /bin/sh syntax is a pain
             ShellQuoted('SHELL ["/bin/bash", "-c"]'),
-        ] + self.install_debian_deps() + [self._change_user()]
-            + [self.workdir(self.option('prefix')),
-            self.create_python_venv(),
-            self.python_venv()])
+        ]
+            + self.install_debian_deps() + [self._change_user()]
+            + [self.workdir(self.option('prefix'))]
+            + self.create_python_venv()
+            + self.python_venv()
+        )
 
     def python_venv(self):
         # To both avoid calling venv activate on each RUN command AND to ensure