Use and enforce python3 shebangs in userscripts

See #6080

misc/userscripts/add-nextcloud-bookmarks

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Behavior:

misc/userscripts/add-nextcloud-cookbook

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Behavior:

misc/userscripts/readability

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Executes python-readability on current page and opens the summary as new tab.
 #

misc/userscripts/ripbang

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Adds DuckDuckGo bang as searchengine.
 #

scripts/dev/misc_checks.py

@@ -313,14 +313,18 @@ def check_userscript_shebangs(_args: argparse.Namespace) -> bool:
             continue
 
         with sub.open('r', encoding='utf-8') as f:
-            shebang = f.readline()
+            shebang = f.readline().rstrip('\n')
         assert shebang.startswith('#!'), shebang
-        binary = shebang.split()[0][2:]
+        shebang = shebang[2:]
 
+        binary = shebang.split()[0]
         if binary not in ['/bin/sh', '/usr/bin/env']:
             bin_name = pathlib.Path(binary).name
             print(f"In {sub}, use #!/usr/bin/env {bin_name} instead of #!{binary}")
             ok = False
+        elif shebang in ['/usr/bin/env python', '/usr/bin/env python2']:
+            print(f"In {sub}, use #!/usr/bin/env python3 instead of #!{shebang}")
+            ok = False
 
     return ok