Add \xHH Unicode escape code to basic strings

[hukkin]

Update tests and merge when https://github.com/toml-lang/toml/pull/796 makes it into a TOML release.

[codecov-commenter]

Codecov Report

Merging #202 (2b195e3) into master (7e563ee) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #202   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          507       509    +2     
  Branches        99       100    +1     
=========================================
+ Hits           507       509    +2     

Impacted Files	Coverage Δ
src/tomli/_parser.py	100.00% <100.00%> (ø)

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