BELGRADE (Reuters) – Serbians will cast their votes on Sunday in a snap election seen as an effort by President Aleksandar Vucic and his populist Serbian Progressive Party (SNS) to secure another four-year term, after two mass shootings earlier this year rattled their popularity. A total of 18 parties and alliances are vying for the support of the 6.5 million-strong electorate for 250 seats in the parliament. The threshold for entering the parliament is 3% of votes. Polls will open at 7 a.m. (0600 GMT) and close at 8 p.m. (1900 GMT). Two mass shootings in May, resulting in 18 deaths, including…