v2.3.0

• Implements FoLiA v2.3
  • Adds support for the explicit form serialisation (proycon/folia#84)
• Bugfixes
  • also serialize when confidence=0 #23
  • fix missing t-ref/@id attribute #19
  • also perform text validation on string elements #15
  • Implemented extra checks for spurious text
  • added requests library as an explicit dependency
  • fixed tests for new xmldiff version