Update maybe_xml dependency 0.3 -> 0.6

[Mingun]

maybe_xml is still about 1.5x--2x faster that quick-xml in most tests, except those:

• rpm_other
• linescore
• test_writer_ident
• players

Here is criterion report: comparison.zip

[codecov-commenter]

Codecov Report

Merging #668 (b9a0fa2) into master (7fbe4bc) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #668   +/-   ##
=======================================
  Coverage   64.75%   64.75%           
=======================================
  Files          37       37           
  Lines       17697    17697           
=======================================
  Hits        11460    11460           
  Misses       6237     6237           

Flag	Coverage Δ
unittests	64.75% <ø> (ø)

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