build: find Python syntax errors in dependencies

As discussed in
https://github.com/nodejs/node/issues/30129#issuecomment-546662351, when
we vendor in code, we own the Syntax Errors in that code.  This PR adds
The `.flake8` config file at the root of this repo puts blinders on the
linting of our dependencies so this test disables that file before
linting.

fixup: allow_failures until dependencies pass

PR-URL: https://github.com/nodejs/node/pull/30143


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>