Update flake8 support

According to the developer documentation, this project adheres to
flake8. Update it so that it actually does this and also add tests for
it to the CI pipeline.

.travis.yml

@@ -36,12 +36,14 @@ install:
 - pip3 install coveralls
 - pip3 install requests
 - pip3 install black
+- pip3 install flake8
 before_script:
 - printf '\nimport coverage\ncoverage.current_coverage = coverage.process_startup()\n'
   >> "/home/travis/virtualenv/python${TRAVIS_PYTHON_VERSION}/lib/python${TRAVIS_PYTHON_VERSION}/sitecustomize.py"
 - rm -f .coverage-report.*
 script:
 - black --check $(git ls-files '*.py')
+- flake8 $(git ls-files '*.py')
 # Pre build the image. This prevents long delays with no output in test cases
 - ./ci/build_image.py $TEST_IMAGE --provider=$DOCKER_PROVIDER --quiet
 - COVERAGE_PROCESS_START=${TRAVIS_BUILD_DIR}/.coveragerc $SH -c "ci/test.py -vb PyrexImage_${DOCKER_PROVIDER}_$(echo $TEST_IMAGE | sed 's/\W/_/g')"

ci/test.py

@@ -466,7 +466,7 @@ def test_optional_bind(self):
         conf["run"]["bind"] += " %s,optional" % missing_bind
         conf.write_conf()
 
-        s = self.assertPyrexContainerShellCommand("test ! -e %s" % missing_bind)
+        self.assertPyrexContainerShellCommand("test ! -e %s" % missing_bind)
 
     def test_bad_bind_option(self):
         temp_dir = tempfile.mkdtemp("-pyrex")

lint

@@ -26,3 +26,4 @@ else
     black --check $(git ls-files '*.py')
 fi
 
+flake8 $(git ls-files '*.py')

pyrex.py

@@ -516,7 +516,7 @@ def prep_container(
     # them.
     env_sock_proxy = config["run"]["envsockproxy"].split()
     for name in set(container_envvars + preserve_env):
-        if not name in os.environ:
+        if name not in os.environ:
             continue
 
         val = os.environ[name]