By Joe Cash BEIJING (Reuters) – Angola’s decision to leave the Organization of the Petroleum Exporting Countries could open the way for Beijing to increase investment in the country’s oil and other sectors, as part of a deepening of decades-old ties. Angola said on Thursday it was leaving OPEC, effective from Jan. 1, following a row with the producer group over the size of its output quota. The decision also follows an agreement signed between China and Angola this month on enhanced cooperation. “China stands out as a pivotal and proven partner,” Angola’s Foreign Minister Tete Antonio said dur…