Kinetics of inhibition of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase by the novel HIV-1-specific nucleoside analogue [2′,5′-Bis-O-(tert-butyldimethylsilyl)-β-D-ribofuranosyl]- 3′-spiro-5″-(4″-amino-1″,2″-oxathiole-2″, 2″-dioxide)thymine (TSAO-T)