setup.cfg: exclude .eggs/** and build/** from pycodestyle linting

Description

• Running the tests locally using ./setup.py test created this egg for me:

  [buildstream]$ find .eggs/
  .eggs/
  .eggs/pytest_runner-4.2-py3.6.egg
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/metadata.json
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/namespace_packages.txt
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/DESCRIPTION.rst
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/WHEEL
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/PKG-INFO
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/requires.txt
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/RECORD
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/top_level.txt
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/entry_points.txt
  .eggs/pytest_runner-4.2-py3.6.egg/EGG-INFO/LICENSE.txt
  .eggs/pytest_runner-4.2-py3.6.egg/ptr.py
  .eggs/README.txt

  ptr.py fails pycodestyle's linting.

• Similarly, running python3 setup.py build creates the build directory which includes buildstream and its dependencies. Some of the dependencies also fails pycodestyle's linting.

We shouldn't lint eggs and build output. We should also ignore them in .gitignore.