The ornamental design for a teabag, as shown and described.
FIG. 1 is a first perspective view of the mesh tea bag showing that all side walls are identical with the exception of the top wall having a string attached to a tag;
FIG. 2 is a second perspective view of the instant invention showing the inside of the mesh tea bag having tea leaves therein with the tea leaves within the bag being visible through the mesh walls; and,
The broken lines showing the tea leaves are for the purpose of illustrating environment, and form no part of the claimed design; and,