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.

Also removed testing for py33 as pip 18.1 demands 3.4.

.travis.yml

@@ -8,6 +8,8 @@ matrix:
         env: TOX_ENV=py27
       - python: 3.3
         env: TOX_ENV=py33
+      - python: 3.4
+        env: TOX_ENV=py34
       - python: 3.5
         env: TOX_ENV=py35
       - python: pypy

libpip2pi/commands.py

@@ -177,6 +177,10 @@ def pip_run_command(pip_args):
     if pip_version < (1, 1):
         raise RuntimeError("pip >= 1.1 required, but %s is installed"
                            %(pip_version, ))
+    # TODO: Remove this once
+    # pip._internal.req.req_tracker.RequirementTracker.cleanup() does it
+    # already.
+    os.environ.pop('PIP_REQ_TRACKER', None)
     res = pip_main(pip_args)
     if res != 0:
         raise PipError("pip failed with status %s while running: %s"

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,33,35}-pip{6,7,8}, pypy
+envlist = py{27,34,35}-pip{6,7,8}, pypy
 
 [testenv]
 deps =