Update phpunit/phpunit requirement from ^9.0 to ^9.1

[dependabot-preview]

Updates the requirements on phpunit/phpunit to permit the latest version.

Changelog

Sourced from phpunit/phpunit's changelog.

9.1.0 - 2020-04-03

Added

• #4061: Implement assertIsNotReadable() as alternative for assertNotIsReadable() with a more readable name
• #4064: Implement assertIsNotWritable() as alternative for assertNotIsWritable() with a more readable name
• #4067: Implement assertDirectoryDoesNotExist() as alternative for assertDirectoryNotExists() with a more readable name
• #4070: Implement assertDirectoryIsNotReadable() as alternative for assertDirectoryNotIsReadable() with a more readable name
• #4073: Implement assertDirectoryIsNotWritable() as alternative for assertDirectoryNotIsWritable() with a more readable name
• #4076: Implement assertFileDoesNotExist() as alternative for assertFileNotExists() with a more readable name
• #4079: Implement assertFileIsNotReadable() as alternative for assertFileNotIsReadable() with a more readable name
• #4082: Implement assertFileIsNotWritable() as alternative for assertFileNotIsWritable() with a more readable name
• #4085: Implement assertMatchesRegularExpression() as alternative for assertRegExp() with a more readable name
• #4088: Implement assertDoesNotMatchRegularExpression() as alternative for assertNotRegExp() with a more readable name
• #4100: Implement failOnIncomplete and failOnSkipped configuration options as well as --fail-on-incomplete and --fail-on-skipped commandline options
• #4130: Canonicalize JSON values in failure message
• #4136: Allow loading PHPUnit extensions via command-line options
• #4148: Support for @preCondition and @postCondition annotations

Changed

• #4039: Deprecate custom test suite loader
• #4062: Deprecate assertNotIsReadable()
• #4065: Deprecate assertNotIsWritable()
• #4068: Deprecate assertDirectoryNotExists()
• #4071: Deprecate assertDirectoryNotIsReadable()
• #4074: Deprecate assertDirectoryNotIsWritable()
• #4077: Deprecate assertFileNotExists()
• #4080: Deprecate assertFileNotIsReadable()
• #4083: Deprecate assertFileNotIsWritable()
• #4086: Deprecate assertRegExp()
• #4089: Deprecate assertNotRegExp()
• #4091: Deprecate assertEqualXMLStructure()
• #4095: Improve performance of StringContains constraint
• #4105: Deprecate multiple test case classes in single file and test case class name differing from filename
• #4141: Deprecate Prophecy integration

Commits

• ef2af93 Prepare release
• 2964a6e Disable test until phpspec/prophecy-phpunit:^2.0 has a stable release
• 7a138d7 Use TestRunner::writeMessage() for test suite loader deprecation warnings
• 10da552 Move printing of test suite loader related warnings from TestSuite to TestRunner
• 9331b00 Fix CS/WS issues
• 60e59de Enable strict interpretation of scalar type declarations
• bbb868d Remove superfluous annotation
• 482cd01 Make deprecation warning message a little less verbose
• a024264 Improve type annotation
• f2ebe30 Update Changelog
• Additional commits viewable in compare view

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