Stricter user ID according to spec

[jaywink]

Make the user ID regexp stricter according to the Matrix specification. Add tests for the function.

See https://matrix.org/docs/spec/appendices#user-identifiers

[codecov-commenter]

Codecov Report

Merging #199 into master will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   89.71%   89.72%   +0.01%     
==========================================
  Files          43       43              
  Lines        6803     6802       -1     
==========================================
  Hits         6103     6103              
+ Misses        700      699       -1     

Impacted Files	Coverage Δ
nio/schemas.py	100.00% <100.00%> (+0.95%)	:arrow_up:

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