build-aux: skip E203 and W503 flake8 checks

build-aux: skip E203 and W503 flake8 checks

The flake8 check W503 does not want a line break before
binary operator. This is contrary to the style that the
'black' formatting tool wants to use. Defer to 'black'
as it is intended to be an opinionated formatting tool
standardizing python code style, and thus not to be
customized per project.

The flake8 check E203 does not want whitespace before
a ':'. This is, however, desirable when indexing array
slices eg

   self.lookahead[skip : skip + 1]

which is a format that 'black' produces.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>