Removing 'PIP_REQ_TRACKER' from the environment variables before exec…

…ution of each pip command.

When populated by one `pip_main` call, the variable remains in the environment properties for all subsequent calls, wreaking havoc since the directory is subsequently deleted.
Should be removed once fixed in `pip`, but will remain until then.

libpip2pi/commands.py

@@ -177,7 +177,11 @@ def pip_run_command(pip_args):
     if pip_version < (1, 1):
         raise RuntimeError("pip >= 1.1 required, but %s is installed"
                            %(pip_version, ))
-    res = pip_main(pip_args)
+    # TODO: Remove this once
+    # pip._internal.req.req_tracker.RequirementTracker.cleanup() does it
+    # already.
+    os.environ.pop('PIP_REQ_TRACKER', None)
+    res = pip_main(['-vvv'] + pip_args)
     if res != 0:
         raise PipError("pip failed with status %s while running: %s"
                        %(res, pip_args))