Update symfony/yaml requirement from ^3.3 to ^4.3

[dependabot-preview]

Updates the requirements on symfony/yaml to permit the latest version.

Changelog

Sourced from symfony/yaml's changelog.

4.3.0

• Using a mapping inside a multi-line string is deprecated and will throw a ParseException in 5.0.

4.2.0

• added support for multiple files or directories in LintCommand

4.0.0

• The behavior of the non-specific tag ! is changed and now forces
  non-evaluating your values.

• complex mappings will throw a ParseException

• support for the comma as a group separator for floats has been dropped, use
  the underscore instead

• support for the !!php/object tag has been dropped, use the !php/object
  tag instead

• duplicate mapping keys throw a ParseException

• non-string mapping keys throw a ParseException, use the Yaml::PARSE_KEYS_AS_STRINGS
  flag to cast them to strings

• % at the beginning of an unquoted string throw a ParseException

• mappings with a colon (:) that is not followed by a whitespace throw a
  ParseException

• the Dumper::setIndentation() method has been removed

• being able to pass boolean options to the Yaml::parse(), Yaml::dump(),
  Parser::parse(), and Dumper::dump() methods to configure the behavior of
  the parser and dumper is no longer supported, pass bitmask flags instead

• the constructor arguments of the Parser class have been removed

• the Inline class is internal and no longer part of the BC promise

• removed support for the !str tag, use the !!str tag instead

• added support for tagged scalars.

  Yaml::parse('!foo bar', Yaml::PARSE_CUSTOM_TAGS);
  // returns TaggedValue('foo', 'bar');

3.4.0

• added support for parsing YAML files using the Yaml::parseFile() or Parser::parseFile() method

• the Dumper, Parser, and Yaml classes are marked as final

• Deprecated the !php/object: tag which will be replaced by the
  !php/object tag (without the colon) in 4.0.

... (truncated)

Commits

• c60ecf5 Merge branch '4.2'
• 6712daf Merge branch '3.4' into 4.2
• 212a27b use behavior instead of behaviour
• 2fb3704 Merge branch '4.2'
• 761fa56 Merge branch '3.4' into 4.2
• 57f1ce8 Apply php-cs-fixer rule for array_key_exists()
• 9744e59 Added deprecation notice when mapping keys are found in multi-line blocks
• 4c7b2f8 Merge branch '4.2' into short-array-master
• d461670 fixed CS
• d1f605b Merge branch '4.1' into 4.2
• Additional commits viewable in compare view

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