alerts: make it compatible with Python3.8 (#838)

Summary:
`re.Pattern[str]` is not valid in Python 3.8

Pull Request resolved: #838

Test Plan:
```
Python 3.8.17 (default, Jul  9 2023, 20:57:35)
[Clang 14.0.3 (clang-1403.0.22.14.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> from typing import Optional
>>> show_after_crashes_regex: Optional[re.Pattern[str]] = None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'type' object is not subscriptable
>>> show_after_crashes_regex: Optional[re.Pattern] = None
>>>
```

Reviewed By: zzl0

Differential Revision: D53903964

fbshipit-source-id: 412f4ad5d6011b0da70c47f31f6c919b9c6a8037

eden/scm/sapling/alerts.py

@@ -17,7 +17,7 @@ class Alert(NamedTuple):
     description: str
     key: str
     show_in_isl: bool
-    show_after_crashes_regex: Optional[re.Pattern[str]]
+    show_after_crashes_regex: Optional[re.Pattern]
 
 
 def parse_alert(ui, key: str, raw_alert: dict) -> Optional[Alert]: