EDMarketConnector: popup for "already running"

* This also refactors code around so that isort and flake8 are happy
  about the module level imports.

EDMarketConnector.py

@@ -18,7 +18,8 @@ from typing import TYPE_CHECKING, Any, Mapping, Optional, Tuple
 
 from constants import applongname, appname, protocolhandler_redirect
 
-def no_other_instance_running() -> bool:  # noqa: CCR001
+if __name__ == "__main__":
+    def no_other_instance_running() -> bool:  # noqa: CCR001
         """
         Ensure only one copy of the app is running under this user account.
 
@@ -86,7 +87,27 @@ def no_other_instance_running() -> bool:  # noqa: CCR001
 
             return EnumWindows(enumwindowsproc, 0)
 
-if __name__ == "__main__":
+        return True
+
+    def already_running_popup():
+        """Create the "already running" popup."""
+        import tkinter as tk
+        from tkinter import ttk
+
+        root = tk.Tk(className=appname.lower())
+
+        frame = tk.Frame(root)
+        frame.grid(row=1, column=0, sticky=tk.NSEW)
+
+        label = tk.Label(frame)
+        label['text'] = 'An EDMarketConnector.exe process was already running, exiting.'
+        label.grid(row=1, column=0, sticky=tk.NSEW)
+
+        button = ttk.Button(frame, text='OK', command=lambda: sys.exit(0))
+        button.grid(row=2, column=0, sticky=tk.S)
+
+        root.mainloop()
+
     if not no_other_instance_running():
         # There's a copy already running.  We want to inform the user by
         # **appending** to the log file, not truncating it.
@@ -97,9 +118,15 @@ if __name__ == "__main__":
             # unbuffered not allowed for text in python3, so use `1 for line buffering
             sys.stdout = sys.stderr = open(join(tempfile.gettempdir(), f'{appname}.log'), mode='a', buffering=1)
 
-        # Logging isn't set up yet
+        # Logging isn't set up yet.
+        # We'll keep this print, but it will be over-written by any subsequent
+        # write by the already-running process.
         print("An EDMarketConnector.exe process was already running, exiting.")
 
+        # To be sure the user knows, we need a popup
+        already_running_popup()
+        # If the user closes the popup with the 'X', not the 'OK' button we'll
+        # reach here.
         sys.exit(0)
 
     # Keep this as the very first code run to be as sure as possible of no
@@ -306,7 +333,6 @@ if TYPE_CHECKING:
         """Fake the l10n translation functions for typing."""
         return x
 
-
 if getattr(sys, 'frozen', False):
     # Under py2exe sys.path[0] is the executable name
     if platform == 'win32':