travis: drop python 3.3 and related workarounds from CI

We still technically support Python 3.3 as our code doesn't use
any Python 3.4 features.
(honestly, there are basically no python 3.4 features worth talking
about -- although of course, you should update just for TLS 1.2 support)
It's just that the whole ecosystem is moving away from py3.3
and I'm tired of making workarounds for pip, virtualenv and friends
simply refusing to accomodate.

.travis.yml

@@ -19,7 +19,6 @@ env:
         PROTOBUF_VERSION=3.4.0
 
 python:
-    - "3.3"
     - "3.4"
     - "3.5"
     - "3.6"
@@ -28,7 +27,6 @@ install:
     # Optimisation: build requirements as wheels, which get cached by Travis
     - pip install "pip>=9.0" wheel  # pip 9.0 understands `python_requires` constraints
     - pip install "setuptools>=38"  # setuptools >= 38 are capable of using prebuilt wheels
-    - pip install "virtualenv<16.0.0"  # virtualenv 16.0.0 drops support for py33 without properly declaring it
     - pip install tox-travis
     - pip install -r requirements-dev.txt
     # protobuf-related dependencies

tox.ini

@@ -10,13 +10,10 @@ deps =
     -rrequirements-dev.txt
 commands =
     # Generate local files
-    python setup.py prebuild
+    python setup.py build
     # Working in the local directory, try to compile all bytecode
     python -m compileall trezorlib/
     # From installed version, smoke-test trezorctl
     trezorctl --help
-    # Run non-device-dependent tests
-    python -m pytest --pyarg trezorlib.tests.unit_tests
-    # This whole thing is a hack, as the tests run against the cwd version of
-    # python-trezor, as opposed to what is installed.
-    # A better solution would be to put `trezorlib` into `src`. But that's not done yet
+    # Run non-device-dependent tests from installed version
+    python -E -m pytest --pyarg trezorlib.tests.unit_tests