Move technical_500 template paths to class-level variables

This allows effortless replacement of the debugging templates
without having to copy-paste the `get_traceback_html` and
`get_traceback_text` functions into a deriving class.

django/views/debug.py

@@ -235,6 +235,10 @@ def get_traceback_frame_variables(self, request, tb_frame):
 
 class ExceptionReporter:
     """Organize and coordinate reporting on exceptions."""
+
+    html_template_path = Path(CURRENT_DIR, 'templates', 'technical_500.html')
+    text_template_path = Path(CURRENT_DIR, 'templates', 'technical_500.txt')
+
     def __init__(self, request, exc_type, exc_value, tb, is_email=False):
         self.request = request
         self.filter = get_exception_reporter_filter(self.request)
@@ -321,14 +325,14 @@ def get_traceback_data(self):
 
     def get_traceback_html(self):
         """Return HTML version of debug 500 HTTP error page."""
-        with Path(CURRENT_DIR, 'templates', 'technical_500.html').open(encoding='utf-8') as fh:
+        with self.html_template_path.open(encoding='utf-8') as fh:
             t = DEBUG_ENGINE.from_string(fh.read())
         c = Context(self.get_traceback_data(), use_l10n=False)
         return t.render(c)
 
     def get_traceback_text(self):
         """Return plain text version of debug 500 HTTP error page."""
-        with Path(CURRENT_DIR, 'templates', 'technical_500.txt').open(encoding='utf-8') as fh:
+        with self.text_template_path.open(encoding='utf-8') as fh:
             t = DEBUG_ENGINE.from_string(fh.read())
         c = Context(self.get_traceback_data(), autoescape=False, use_l10n=False)
         return t.render(c)