Fix diff_for_humans crashing for the zh locale

zh/custom.py used a NAMED placeholder in "after"/"before" ("{time}后",
"{time}前") while DifferenceFormatter.format() substitutes positionally
(difference_formatter.py:115,159 call locale.get(key).format(time)).
So the two-datetime form raises KeyError: 'time' for every unit:

    a.diff_for_humans(b, locale="zh")   ->  KeyError: 'time'

All 28 other locales use "{0}". zh is switched to match; no code change.
The relative-to-now form was unaffected because it reads the CLDR
translations table rather than these custom entries, and there was no
tests/localization/test_zh.py at all, so nothing exercised this path.

Adds that test file, covering the relative-to-now form and the
two-datetime form that was crashing.

Author: dylanpulver

src/pendulum/locales/zh/custom.py

@@ -6,8 +6,8 @@
 
 translations = {
     # Relative time
-    "after": "{time}后",
-    "before": "{time}前",
+    "after": "{0}后",
+    "before": "{0}前",
     # Date formats
     "date_formats": {
         "LTS": "Ah点m分s秒",

tests/localization/test_zh.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import pendulum
+
+
+locale = "zh"
+
+
+def test_diff_for_humans():
+    with pendulum.travel_to(pendulum.datetime(2016, 8, 29), freeze=True):
+        diff_for_humans()
+
+
+def diff_for_humans():
+    d = pendulum.now().subtract(seconds=1)
+    assert d.diff_for_humans(locale=locale) == "1秒钟前"
+
+    d = pendulum.now().subtract(minutes=2)
+    assert d.diff_for_humans(locale=locale) == "2分钟前"
+
+    d = pendulum.now().subtract(hours=2)
+    assert d.diff_for_humans(locale=locale) == "2小时前"
+
+    d = pendulum.now().subtract(days=2)
+    assert d.diff_for_humans(locale=locale) == "2天前"
+
+    d = pendulum.now().subtract(weeks=2)
+    assert d.diff_for_humans(locale=locale) == "2周前"
+
+    d = pendulum.now().subtract(months=2)
+    assert d.diff_for_humans(locale=locale) == "2个月前"
+
+    d = pendulum.now().subtract(years=2)
+    assert d.diff_for_humans(locale=locale) == "2年前"
+
+    d = pendulum.now().add(seconds=1)
+    assert d.diff_for_humans(locale=locale) == "1秒钟后"
+
+    d = pendulum.now().add(seconds=1)
+    d2 = pendulum.now()
+    assert d.diff_for_humans(d2, locale=locale) == "1秒钟后"
+    assert d2.diff_for_humans(d, locale=locale) == "1秒钟前"
+
+    assert d.diff_for_humans(d2, True, locale=locale) == "1秒钟"
+    assert d2.diff_for_humans(d.add(seconds=1), True, locale=locale) == "2秒钟"