[lint] add a .isort.cfg file contrib/buildbot

Summary:
On some systems ISort seems to think build.py and server.py are third-party package rather than local modules.
This is clearly a limitation of ISort, because the python import order respects the order of the paths listed in the sys.path list which by default has the current working directory as first element (https://docs.python.org/3/library/sys.html#sys.path).

The third-party versions of build and server seem to be installed as indirect Electrum ABC requirements, which makes this problem apparent for build, but it was already doing the wrong sorting for server.

Test Plan: arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14106