configure: Bump the minimum required Python version to 3.6

All our supported build platforms have Python 3.6 or newer nowadays, and
there are some useful features in Python 3.6 which are not available in
3.5 yet (e.g. the type hint annotations which will allow us to statically
type the QAPI parser), so let's bump the minimum Python version to 3.6 now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200923162908.95372-1-thuth@redhat.com>
Message-Id: <20200925154027.12672-16-alex.bennee@linaro.org>