Add flake8 check for pygen (chipsalliance#589)

Author: danghai

.flake8

@@ -0,0 +1,10 @@
+[flake8]
+
+# Maximum line length
+max-line-length = 100
+
+ignore =
+  # Ignore unexpected spaces around keyword / parameter equals
+  E251,
+  # Do not complain about line breaks after operators
+  W504

.travis.yml

@@ -2,8 +2,6 @@ language: python
 dist: xenial
 matrix:
     include:
-        - python: 3.5
-          env: TOX_ENV=py35
         - python: 3.6
           env: TOX_ENV=py36
         - python: 3.7
@@ -15,3 +13,4 @@ install:
 script:
     - sphinx-build -E -W -b linkcheck docs/source build
     - pip uninstall -y riscv-dv
+    - flake8 --show-source pygen/pygen_src/ --config=.flake8

requirements.txt

@@ -6,3 +6,4 @@ sphinxcontrib-log-cabinet
 sphinx-issues
 sphinx_rtd_theme
 rst2pdf
+flake8