lint: Replace pycodestyle with Flake8.

Flake8 combines pycodestyle with pyflakes and automatically gives us
support for noqa comments, parallelism, configuration files, plugins,
and easy editor integration.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

.flake8

@@ -0,0 +1,59 @@
+[flake8]
+ignore =
+    # Each of these rules are ignored for the explained reason.
+
+    # "multiple spaces before operator"
+    # There are several typos here, but also several instances that are
+    # being used for alignment in dict keys/values using the `dict`
+    # constructor. We could fix the alignment cases by switching to the `{}`
+    # constructor, but it makes fixing this rule a little less
+    # straightforward.
+    E221,
+
+    # 'missing whitespace around arithmetic operator'
+    # This should possibly be cleaned up, though changing some of
+    # these may make the code less readable.
+    E226,
+
+    # "unexpected spaces around keyword / parameter equals"
+    # Many of these should be fixed, but many are also being used for
+    # alignment/making the code easier to read.
+    E251,
+
+    # "block comment should start with '#'"
+    # These serve to show which lines should be changed in files customized
+    # by the user. We could probably resolve one of E265 or E266 by
+    # standardizing on a single style for lines that the user might want to
+    # change.
+    E265,
+
+    # "too many leading '#' for block comment"
+    # Most of these are there for valid reasons.
+    E266,
+
+    # "expected 2 blank lines after class or function definition"
+    # Zulip only uses 1 blank line after class/function
+    # definitions; the PEP-8 recommendation results in super sparse code.
+    E302, E305,
+
+    # "module level import not at top of file"
+    # Most of these are there for valid reasons, though there might be a
+    # few that could be eliminated.
+    E402,
+
+    # "line too long"
+    # Zulip is a bit less strict about line length, and has its
+    # own check for this (see max_length)
+    E501,
+
+    # "line break before binary operator"
+    # This was obsoleted in favor of the opposite W504.
+    W503,
+
+    # "do not assign a lambda expression, use a def"
+    # Fixing these would probably reduce readability in most cases.
+    E731,
+
+exclude =
+    # third-party
+    zulip/integrations/perforce/git_p4.py,