Remove support for python 3.18 and 3.19.

update changelog

update nltk from 3.8.0 to 3.9.1

• Update copyright details.

• Always skip doubled special characters, not just in case the cue
  starters are doubled.

• Fix an issue with WebVTT writer text positioning on break inside a
  cue.
• Prevent creating a repositioning command to the same coordinates.

• Mid-row codes only add spaces only if there isn't one before.
• Mid-row codes add spaces only if they affect the text in the same
  row (not adding if it follows break or PACS).
• Remove spaces to the end of the lines.
• Close italics on receiving another style setting command.
• Throw an CaptionReadNoCaptions error in case of empty input file are
  provided.
• Ignore repositioning commands which are not followed by any text
  before breaks.
• Mid-row codes will not add the space if it is in front of
  punctuation.
• Fix a bug with background codes when the InstructionNodeCreator
  collection is empty.
• Fix a bug WebVTT writer adding double line breaks.

pinned nltk to 3.8.0

• A space should not be placed before a mid row code if it follows a
  PAC command or a Tab Offset
• The backspace command should be treated like other commands and
  duplicates should be skipped if PAC commands are duplicated
• Prevent webvtt writer from creating a new cue in case of line break
• In case of style setting PAC which also breaks the line, we add the
  break first, then the style tag

2.2.10 ^^^^^ - Yanked.

A space should not be placed before a mid row code if it follows a
PAC command or a Tab Offset
The backspace command should be treated like other commands and
duplicates should be skipped if PAC commands are duplicated