Safeguard against VMS version numbers in tree names returned by uproot

[gordonwatts]

• New version of uproot/awkward returns the version number for tree keys - but can't handle them in an uproot.open. Strip off the version number if it is there.
  • Classic example of VMS living on!
• Revise default configuration for backend name to reflect how ServiceX is usually configured, moving from atlasxaod to atlasr21.

Fixes #326

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c61bdad) 98.45% compared to head (ad02e78) 98.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files          12       12           
  Lines        1166     1168    +2     
=======================================
+ Hits         1148     1150    +2     
  Misses         18       18           

Flag	Coverage Δ
unittests-3.10	98.37% <100.00%> (+<0.01%)	⬆️
unittests-3.11	98.37% <100.00%> (+<0.01%)	⬆️
unittests-3.7	98.28% <100.00%> (+<0.01%)	⬆️
unittests-3.8	98.37% <100.00%> (+<0.01%)	⬆️
unittests-3.9	98.37% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
servicex/data_conversions.py	98.27% <100.00%> (+0.03%)	⬆️
servicex/servicex.py	100.00% <ø> (ø)
servicex/servicex_config.py	100.00% <100.00%> (ø)

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