Turn on py36 at Travis

Drops the old py35 job, per @exarkun:

https://github.com/tahoe-lafs/tahoe-lafs/pull/732#discussion_r449615599

Also bumps Ubuntu at Travis so we can get 3.8 eventually. I first went
to 3.8 before dropping back to 3.6 as our initial target. Trusty on
Travis does include 3.6, but since we want 3.8 "pretty soon," and the OS
bump ended up being tricky (see below), let's go ahead and keep the OS
bump. Xenial (16.04) is the current default at Travis, and it does have
3.8 available:

https://docs.travis-ci.com/user/languages/python/#python-versions

The tricky bug is that different versions of virtualenv have different
seeding algorithms: https://discuss.python.org/t/-/4146). CI puts us
several layers deep in virtualenv-ception and I didn't fully unravel the
whole thing, but starting with a modern virtualenv seems to work around
the issue.

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: python
 cache: pip
-dist: trusty
+dist: xenial
 before_cache:
   - rm -f $HOME/.cache/pip/log/debug.log
 git:
@@ -16,19 +16,15 @@ install:
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then export PATH=$HOME/Library/Python/2.7/bin:$PATH; fi
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then wget https://bootstrap.pypa.io/get-pip.py && sudo python ./get-pip.py; fi
   - pip list
-  - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then pip install --user --upgrade codecov tox setuptools; fi
-  - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then pip install --upgrade codecov tox setuptools; fi
+  - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then pip install --user --upgrade codecov tox setuptools virtualenv; fi
+  - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then pip install --upgrade codecov tox setuptools virtualenv; fi
   - echo $PATH; which python; which pip; which tox
   - python misc/build_helpers/show-tool-versions.py
 
 script:
   - |
     set -eo pipefail
-    if [ "${T}" = "py35" ]; then
-      python3 -m compileall -f -x tahoe-depgraph.py .
-    else
-      tox -e ${T}
-    fi
+    tox -e ${T}
     # To verify that the resultant PyInstaller-generated binary executes
     # cleanly (i.e., that it terminates with an exit code of 0 and isn't
     # failing due to import/packaging-related errors, etc.).
@@ -69,9 +65,8 @@ matrix:
     python: '2.7'
     env: T=pyinstaller LANG=en_US.UTF-8
     language: generic  # "python" is not available on OS-X
-  # this is a "lint" job that checks for python3 compatibility
   - os: linux
-    python: '3.5'
-    env: T=py35
+    python: '3.6'
+    env: T=py36
 
   fast_finish: true