(Reuters) – Boris Johnson is stepping down as British prime minister, forcing the ruling Conservative Party to choose a new leader who will be announced on Sept. 5. The winner will inherit a parliamentary majority and will become the next prime minister. WEDNESDAY JULY 20 – FINAL SELECTION BALLOT Conservative members of parliament chose former finance minister Rishi Sunak and Foreign Secretary Liz Truss to go through to the final stage – a vote by around 200,000 party members. JULY-SEPTEMBER – THE RUN-OFF The two candidates campaign among party members to secure their votes. The Conservative P…