Added Gitea fetcher

updates_notifier/fetchers/gitea_fetcher.py

+from .project_fetcher import ProjectFetcher
+from .github_fetcher import GithubFetcher
+
+
+class GiteaFetcher(GithubFetcher):
+    def __init__(self, feed_id: int, name: str, gitea_host: str, repository: str):
+        ProjectFetcher.__init__(self, feed_id, name)
+
+        self.gitea_host = gitea_host
+        self.repository = repository
+
+    def _get_releases_url(self) -> str:
+        return f"https://{self.gitea_host}/api/v1/repos/{self.repository}/releases"