Add sage.misc.latex.pdf to save the image of objects to pdf

[kwankyu]

Thus

sage: from sage.misc.latex import pdf
sage: pdf(pi, 'test.pdf')

works. This is a companion to the existing sage.misc.latex.png, which saves png image of objects. Of course, pdf image is scalable and hence more suitable for inclusion into latex documents.

Also make view() more robust by cleaning up the temporary file after waiting for a few seconds, which is enough time for the viewer to open. For example,

sage: view(pi)  # show pi

which sometimes fails because the temporary file is removed before the viewer opens the file: https://groups.google.com/g/sage-support/c/YfyaZxc8Z6k

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