build(deps-dev): bump guzzlehttp/psr7 from 1.7.0 to 1.8.1

Bumps [guzzlehttp/psr7](https://github.com/guzzle/psr7) from 1.7.0 to 1.8.1.
- [Release notes](https://github.com/guzzle/psr7/releases)
- [Changelog](https://github.com/guzzle/psr7/blob/master/CHANGELOG.md)
- [Commits](guzzle/psr7@1.7.0...1.8.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -21,7 +21,7 @@
     "require-dev": {
         "composer/package-versions-deprecated": "1.11.99.1",
         "friendsofphp/php-cs-fixer": "^2.18",
-        "guzzlehttp/psr7": "^1.7",
+        "guzzlehttp/psr7": "^1.8",
         "http-interop/http-factory-guzzle": "^1.0",
         "infection/infection": "^0.11.5",
         "php-http/mock-client": "^1.1",