fix: make readalongs prepare Windows compatible

 - open the input file in utf-8 mode
 - don't open the input file until we need it, so we don't keep it open
   in case of error (causes unit testing errors when file handles are
   not closed so that temp files cannot be deleted)
 - the click file name for stdin is "-" on Windows, "<stdin>" on Linux

readalongs/cli.py

@@ -304,7 +304,7 @@ def epub(**kwargs):
     context_settings=CONTEXT_SETTINGS,
     short_help="Convert a plain text file into the XML format for alignment.",
 )
-@click.argument("plaintextfile", type=click.File("r", encoding="utf8"))
+@click.argument("plaintextfile", type=click.File("r", encoding="utf8", lazy=True))
 @click.argument("xmlfile", type=click.Path(), required=False, default="")
 @click.option("-d", "--debug", is_flag=True, help="Add debugging messages to logger")
 @click.option(
@@ -344,7 +344,8 @@ def prepare(**kwargs):
     out_file = kwargs["xmlfile"]
     if not out_file:
         out_file = get_click_file_name(input_file)
-        if out_file == "<stdin>":  # actual intput_file.name when cli input is "-"
+        print(f"input_file={out_file}")
+        if out_file in ("<stdin>", "-"):  # intput_file.name is <stdin> on Linux, - on Windows, when cli input is "-"
             out_file = "-"
         else:
             if out_file.endswith(".txt"):