Make pgettext search plurals when translation is not found (#1085)

pgettext can now find the following translation
when using `pgettext("ctx", "foo")`:
```
msgctxt "ctx"
msgid "foo"
msgid_plural "foos"
msgstr[0] "foo translated"
```

The upstream CPython PR is  python/cpython#107118

Author: tomasr8

babel/support.py

@@ -466,10 +466,12 @@ def pgettext(self, context: str, message: str) -> str | object:
         missing = object()
         tmsg = self._catalog.get(ctxt_msg_id, missing)
         if tmsg is missing:
-            if self._fallback:
-                return self._fallback.pgettext(context, message)
-            return message
-        return tmsg
+            tmsg = self._catalog.get((ctxt_msg_id, self.plural(1)), missing)
+        if tmsg is not missing:
+            return tmsg
+        if self._fallback:
+            return self._fallback.pgettext(context, message)
+        return message
 
     def lpgettext(self, context: str, message: str) -> str | bytes | object:
         """Equivalent to ``pgettext()``, but the translation is returned in the

tests/test_support.py

@@ -71,6 +71,14 @@ def test_pgettext(self):
         self.assertEqualTypeToo('Voh', self.translations.gettext('foo'))
         self.assertEqualTypeToo('VohCTX', self.translations.pgettext('foo',
                                                                      'foo'))
+        self.assertEqualTypeToo('VohCTX1', self.translations.pgettext('foo',
+                                                                      'foo1'))
+
+    def test_pgettext_fallback(self):
+        fallback = self.translations._fallback
+        self.translations._fallback = support.NullTranslations()
+        assert self.translations.pgettext('foo', 'bar') == 'bar'
+        self.translations._fallback = fallback
 
     def test_upgettext(self):
         self.assertEqualTypeToo('Voh', self.translations.ugettext('foo'))