The ornamental design for a teabag, as shown and described.
FIG. 1 is a perspective view of a tea bag showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof shown in environments of use.
The broken line showing of portions of the tea bag is included for the purpose of illustrating environmental structure forms no part of the claimed design.