Prevent usage with flake8 v3

flake8 v3 internals are quite different,
requiring major surgery to be compatible.
At present, prevent usage with v3.

.travis.yml

@@ -16,7 +16,7 @@ env:
     # Workaround for https://github.com/jayvdb/flake8-putty/issues/16
     - VIRTUALENV_DOWNLOAD=no
   matrix:
-    - TOXENV=v25x,v241,v230,v224,v220,v210,v200
+    - TOXENV=v26x,v255,v241,v230,v224,v220,v210,v200
     - TOXENV=green
     - TOXENV=pytest
 

setup.py

@@ -65,7 +65,9 @@ def get_long_description():
     author_email='jayvdb@gmail.com',
     url='https://github.com/jayvdb/flake8-putty',
     install_requires=[
-        'flake8>=2',  # extensions were introduced in 2.0
+        # extensions were introduced in 2.0
+        # flake8 v3 internals are very different
+        'flake8>=2,<3',
     ],
     extras_require=extras,
     license='MIT',

tox.ini

@@ -11,12 +11,12 @@ passenv =
     APPVEYOR_REPO_COMMIT APPVEYOR_REPO_BRANCH APPVEYOR_BUILD_VERSION
     APPVEYOR_JOB_ID APPVEYOR_PULL_REQUEST_NUMBER
 commands =
-    flakes,v25x,v250,v241,v240,v230,v224,v220,v210,v200: flake8 --version
+    flakes,v26x,v255,v241,v240,v230,v224,v220,v210,v200: flake8 --version
     # --jobs 32 is default, and is ignored on Microsoft Windows
     # See https://github.com/travis-ci/travis-ci/issues/4696
-    v250,v241,v240,v230,v224: flake8 --jobs 32 {posargs}
+    v255,v241,v240,v230,v224: flake8 --jobs 32 {posargs}
     # Use --jobs 1 to force single process configuration
-    flakes,v25x: flake8 --jobs 1 {posargs}
+    flakes,v26x: flake8 --jobs 1 {posargs}
     # use --jobs 2 to force BaseQReport to reuse a subprocess
     # Note v220 with --jobs fails on Windows.
     v220: flake8 --jobs 2 {posargs}
@@ -34,8 +34,8 @@ deps =
     pytest: pytest-runner
     pytest: pytest-cov
     pytest: codecov
-    v25x: flake8>2.5.0
-    v250: flake8==2.5.0
+    v26x: flake8>=2.6.2,<2.7
+    v255: flake8==2.5.5
     v241: flake8==2.4.1
     v240: flake8==2.4.0
     v230: flake8==2.3.0