doc: Don't limit ourselves to flake8 2.x

There was a bug when using hacking with flake8 3.x. This bug has since
been resolved [1], meaning we no longer need to call out the need to use
the older version of flake8.

[1] https://review.openstack.org/#/c/335965/

Signed-off-by: Stephen Finucane <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

.travis/linux-prepare.sh

@@ -3,7 +3,4 @@
 git clone git://git.kernel.org/pub/scm/devel/sparse/chrisl/sparse.git
 cd sparse && make && make install && cd ..
 
-# Incompatibility between flake8 3.0.x and the hacking plugin:
-# https://gitlab.com/pycqa/flake8/issues/153
-# https://bugs.launchpad.net/hacking/+bug/1607942
-pip install --disable-pip-version-check --user six "flake8<3.0" hacking
+pip install --disable-pip-version-check --user six flake8 hacking

Documentation/intro/install/general.rst

@@ -147,14 +147,10 @@ following to obtain better warnings:
 
 - clang, version 3.4 or later
 
-- flake8, version 2.X, along with the hacking flake8 plugin (for Python code).
-  The automatic flake8 check that runs against Python code has some warnings
-  enabled that come from the "hacking" flake8 plugin. If it's not installed,
-  the warnings just won't occur until it's run on a system with "hacking"
-  installed. Note that there are problems with flake8 3.0 and the "hacking"
-  plugin. To ensure you get flake8 2.X, you can use::
-
-      $ pip install 'flake8<3.0'
+- flake8 along with the hacking flake8 plugin (for Python code). The automatic
+  flake8 check that runs against Python code has some warnings enabled that
+  come from the "hacking" flake8 plugin. If it's not installed, the warnings
+  just won't occur until it's run on a system with "hacking" installed.
 
 You may find the ovs-dev script found in ``utilities/ovs-dev.py`` useful.