Pane(fix[display_popup]): use fused -e flag format for consistency

why: All other methods (split, respawn, new_session, new_window) use
the fused form (f"-e{k}={v}",) for environment flags. display_popup
was the sole outlier using the separated form ("-e", f"{k}={v}").
what:
- Change ("-e", f"{k}={v}") to (f"-e{k}={v}",)

Author: tony

src/libtmux/pane.py

@@ -1271,7 +1271,7 @@ def display_popup(
         if environment:
             if has_gte_version("3.3", tmux_bin=self.server.tmux_bin):
                 for k, v in environment.items():
-                    tmux_args += ("-e", f"{k}={v}")
+                    tmux_args += (f"-e{k}={v}",)
             else:
                 warnings.warn(
                     "environment requires tmux 3.3+, ignoring",