Emerges fully-formed and working thanks to lots of help from Louis and Chris

.env.example

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+FORGEJO_INSTANCE=https://git.example.com
+FORGEJO_API_TOKEN=create-at-/user/settings/applications

move_issue.py

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+import os
+import sys
+from dotenv import load_dotenv
+import requests
+
+load_dotenv()
+
+FORGEJO_INSTANCE = os.getenv("FORGEJO_INSTANCE", "https://codeberg.org")
+FORGEJO_API_TOKEN = os.getenv("FORGEJO_API_TOKEN")
+
+API_GET_ISSUE = "{instance}/api/v1/repos/{owner}/{repo}/issues/{index}"
+API_CREATE_ISSUE = "{instance}/api/v1/repos/{owner}/{repo}/issues"
+
+def move_issue(source_owner, source_repo, issue_number, destination_owner, destination_repo):
+    get_issue_url = API_GET_ISSUE.format(instance=FORGEJO_INSTANCE, owner=source_owner, repo=source_repo, index=issue_number)
+    # print(get_issue_url)
+    response = requests.get(get_issue_url, params={"access_token": FORGEJO_API_TOKEN})
+    json_response = response.json()
+
+    # Not 100% certain this can ever give a message, but create below does on error
+    message = json_response.pop('message', False)
+    if message:
+        print(message)
+
+    if json_response and 'milestone' in json_response:
+        print(f"We have removed the milestone {json_response['milestone']['title']} from this issue (milestones are specific to repositories).")
+        del json_response['milestone']
+
+    print(f"Creating issue {json_response['title']} now…")
+
+    create_issue_url = API_CREATE_ISSUE.format(instance=FORGEJO_INSTANCE, owner=destination_owner, repo=destination_repo)
+    # print(create_issue_url)
+    headers = {"Authorization": f"token {FORGEJO_API_TOKEN}"}
+    created_response = requests.post(create_issue_url, headers=headers, data=json_response)
+
+    json_response = created_response.json()
+    message = json_response.pop('message', False)
+    if message:
+        print(message)
+    else:
+        print(f"Issue created: {json_response['html_url']}")
+
+if __name__=="__main__":
+    # '0' comes in as move_issue.py when this is called as `python move_issue.py`
+    source_owner = sys.argv[1]
+    source_repo = sys.argv[2]
+    issue_number = sys.argv[3]
+    destination_owner = sys.argv[4]
+    destination_repo = sys.argv[5]
+
+    move_issue(source_owner, source_repo, issue_number, destination_owner, destination_repo)