pep8 package was renamed to pycodestyle to reduce confusion.

Disabled warnings:
W504 - Line break after binary operator (seems people like this)
W605 - Invalid escape sequence (some of our regexes use these)

This helps identifying what tests are slow or if the CI got stuck on
some of them

This has been done in a way that I hope might be able to be upstreamed
in to versioneer. This is not garanteed but it would be good if it
could be.

Please see https://github.com/warner/python-versioneer/pull/186 and
https://github.com/wsalmonct/python-versioneer/tree/feature/regexTags
for further details.

This is causing pytest to try to import the recently added app1.py
from the pip related tests. This fails because it's not meant to be
imported outside of the sandbox environment.

This allows code generation with ./setup.py build_grpc

Using setuptools_scm had a couple of bad problems:

  o Unexpected versioning semantics, setuptools_scm would
    increment the micro version by itself in the case that
    we derive a version number from something that is not a tag,
    making the assumption that we are "leading up to" the next
    micro version.

    People mostly dont expect this.

  o When installing in developer mode, i.e. with `pip3 install --user -e .`,
    then we were always picking the generated version at install time
    and never again dynamically resolving it.

    Many of our users install this way and update through git, so it's
    important that we report more precise versions all the time.

This commit needs to make a series of changes at the same time:

  o Adds versioneer.py to the toplevel, this is used by setup.py
    for various activities.

    This is modified only to inform the linter to skip

  o Adds buildstream/_version.py, which is generated by versioneer
    and gives us the machinery to automatically derive the correct version

    This is modified only to inform the linter to skip

  o Adds a .gitattributes file which informs git to substitute
    the buildstream/_version.py file, this is just to ensure that
    the versioning output would work if ever we used `git archive`
    to release a tarball.

  o Modifies setup.py and setup.cfg for versioneer

  o Modifies utils.py and _frontend/cli.py such as to avoid importing
    the derived version when running bash completion mode, we dont
    derive the version at completion time because this can result
    in running a subprocess (when running in developer install mode)
    and is an undesirable overhead.

  o Updates tests/frontend/version.py to expect changed version output