email_notifications: Give absolute path to html2text.

Our uWSGI configuration doesn’t correctly activate our virtualenv.  We
should investigate that, but until we do, we need to invoke html2text
by an absolute path.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/email_notifications.py

@@ -1,8 +1,10 @@
 # See https://zulip.readthedocs.io/en/latest/subsystems/notifications.html
 
 import logging
+import os
 import re
 import subprocess
+import sys
 from collections import defaultdict
 from datetime import timedelta
 from email.headerregistry import Address
@@ -705,7 +707,9 @@ def enqueue_welcome_emails(user: UserProfile, realm_creation: bool = False) -> N
 
 def convert_html_to_markdown(html: str) -> str:
     # html2text is GPL licensed, so run it as a subprocess.
-    markdown = subprocess.check_output(["html2text"], input=html, text=True).strip()
+    markdown = subprocess.check_output(
+        [os.path.join(sys.prefix, "bin", "html2text")], input=html, text=True
+    ).strip()
 
     # We want images to get linked and inline previewed, but html2text will turn
     # them into links of the form `![](http://foo.com/image.png)`, which is