remove python 3.13 workflow pin when pyobjcryst and diffpy.srreal support python 3.14

[cadenmyers13]

problem

Pinned python version to 3.13 in tests-on-pr.yml because diffpy.srreal and pyobjcryst dependencies don't support python 3.14 yet. See conversation on this PR: #65 (comment)

solution

remove the pin when pyobjcryst and diffpy.srreal support python 3.14