fix cgi deprecation warning

[hboetes]

I kept noticing this deprecation warning:

/home/han/.virtualenv/flexget/lib/python3.12/site-packages/flexget/plugins/cli/inject.py:2: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13
import cgi

So I started digging around in the docs and came up with the following patch:

%  diff -u /home/han/.virtualenv/flexget/lib/python3.12/site-packages/flexget/plugins/cli/inject.py.orig /home/han/.virtualenv/flexget/lib/python3.12/site-packages/flexget/plugins/cli/inject.py
--- /home/han/.virtualenv/flexget/lib/python3.12/site-packages/flexget/plugins/cli/inject.py.orig	2024-02-14 18:56:24.723822371 +0100
+++ /home/han/.virtualenv/flexget/lib/python3.12/site-packages/flexget/plugins/cli/inject.py	2024-02-14 18:59:18.184871193 +0100
@@ -1,5 +1,5 @@
 import argparse
-import cgi
+from email.header import Header
 import random
 import string
 
@@ -23,7 +23,7 @@
     if options.url and not options.title:
         # Attempt to get a title from the URL response's headers
         try:
-            value, params = cgi.parse_header(
+            value, params = Header(
                 requests.head(options.url).headers['Content-Disposition']
             )
             options.title = params['filename']

I have no idea if this really fixes the issue, or just the symptom, so please double triple check before applying this patch.