gh-69714: Add missing test cases for custom locale in calendar.Locale{Text|HTML}Calendar

There are 3 paths to use `locale` argument in
`calendar.Locale{Text|HTML}Calendar.__init__(..., locale=None)`:
(1) `locale=None` -- denotes the "default locale"[1]
(2) `locale=""` -- denotes the native environment
(3) `locale=other_valid_locale` -- denotes a custom locale

So far case (2) is covered and case (1) is in 78935da (same branch).
This commit adds a remaining case (3).

[1] In the current implementation, this translates into the following
approach:

GET current locale
IF current locale == "C" THEN
  SET current locale TO ""
  GET current locale
ENDIF

Author: bxsx

Lib/test/test_calendar.py

@@ -571,6 +571,17 @@ def test_locale_calendars(self):
         self.assertEqual(len(local_weekday_abbr), 3)
         self.assertGreaterEqual(len(local_month), 10)
 
+        cal = calendar.LocaleTextCalendar(locale='C')
+        local_weekday = cal.formatweekday(1, 10)
+        local_weekday_abbr = cal.formatweekday(1, 3)
+        local_month = cal.formatmonthname(2010, 10, 10)
+        self.assertIsInstance(local_weekday, str)
+        self.assertIsInstance(local_weekday_abbr, str)
+        self.assertIsInstance(local_month, str)
+        self.assertEqual(len(local_weekday), 10)
+        self.assertEqual(len(local_weekday_abbr), 3)
+        self.assertGreaterEqual(len(local_month), 10)
+
         cal = calendar.LocaleHTMLCalendar(locale=None)
         local_weekday = cal.formatweekday(1)
         local_month = cal.formatmonthname(2010, 10)
@@ -583,6 +594,12 @@ def test_locale_calendars(self):
         self.assertIsInstance(local_weekday, str)
         self.assertIsInstance(local_month, str)
 
+        cal = calendar.LocaleHTMLCalendar(locale='C')
+        local_weekday = cal.formatweekday(1)
+        local_month = cal.formatmonthname(2010, 10)
+        self.assertIsInstance(local_weekday, str)
+        self.assertIsInstance(local_month, str)
+
         new_october = calendar.TextCalendar().formatmonthname(2010, 10, 10)
         self.assertEqual(old_october, new_october)