pin sunpy dependency to <5.2.0

sunpy's HelioviewerClient is deprecated will be removed in favor of hvpy (https://github.com/Helioviewer-Project/python-api)

fall back to mirror if helioviewer.org is offline (fixes #5)

upgrade SunPy to 2.1.0, remove workaround which is now included in Su…

…nPy (sunpy/sunpy#4561)

update screenshot

update note in README.md