Don't use fstrings to keep support with python<3.7

fixes #131

toot/tui/app.py

@@ -26,7 +26,7 @@ class Header(urwid.WidgetWrap):
         self.text = urwid.Text("")
         self.cols = urwid.Columns([
             ("pack", urwid.Text(('header_bold', 'toot'))),
-            ("pack", urwid.Text(('header', f' | {user.username}@{app.instance}'))),
+            ("pack", urwid.Text(('header', ' | {}@{}'.format(user.username, app.instance)))),
             ("pack", self.text),
         ])
 

toot/tui/compose.py

@@ -16,9 +16,9 @@ class StatusComposer(urwid.Frame):
         self.in_reply_to = in_reply_to
         text, edit_pos = '', None
         if in_reply_to is not None:
-            text = f'@{in_reply_to.account} '
+            text = '@{} '.format(in_reply_to.account)
             edit_pos = len(text)
-            mentions = [f'@{m["acct"]}' for m in in_reply_to.mentions]
+            mentions = ['@{}'.format(m["acct"]) for m in in_reply_to.mentions]
             if mentions:
                 text += '\n\n{}'.format(' '.join(mentions))
         self.content_edit = EditBox(edit_text=text, edit_pos=edit_pos,