Python 3.9 finally removed sys.setcheckinterval().  While the package
apparently tried to account for that, the logic is flawed and the second
getattr() raises an AttributeError even if its result is never used.
This caused tests to fail:

      File "/tmp/tqdm/tqdm/tests/tests_tqdm.py", line 126, in pretest
        getattr(sys, 'setswitchinterval', getattr(sys, 'setcheckinterval'))(100)
    AttributeError: module 'sys' has no attribute 'setcheckinterval'

Refactor the code into a try/except construct that does not execute
the setcheckinterval() branch unless setswitchinterval() is actually
missing.  While at it, scale the arguments a bit -- the current version
used either 100 instructions or 100 seconds that were rather of very
different magnitudes.

- fixes #979