Since feedparser messes with Python’s internals by assigning
to __code__ it needs to import SGMLParseError too. It also
expects SGMLParseError, which was turned into AssertionError by
https://github.com/python/cpython/commit/e34bbfd61f405eef89e8aa50672b0b25022de320

--- feedparser-6.0.10/feedparser/sgml.py.orig	2023-03-18 09:24:50.976316932 +0100
+++ feedparser-6.0.10/feedparser/sgml.py	2023-03-18 09:26:32.971928811 +0100
@@ -28,6 +28,7 @@
 import re
 
 import sgmllib
+from sgmllib import SGMLParseError
 
 __all__ = [
     'sgmllib',
@@ -41,6 +42,7 @@
     'shorttagopen',
     'starttagopen',
     'endbracket',
+    'SGMLParseError',
 ]
 
 # sgmllib defines a number of module-level regular expressions that are
--- feedparser-6.0.10/feedparser/html.py.orig	2023-03-18 09:32:03.647114745 +0100
+++ feedparser-6.0.10/feedparser/html.py	2023-03-18 09:46:05.021142671 +0100
@@ -349,7 +349,7 @@
 
         try:
             return sgmllib.SGMLParser.parse_declaration(self, i)
-        except sgmllib.SGMLParseError:
+        except AssertionError:
             # Escape the doctype declaration and continue parsing.
             self.handle_data('<')
             return i+1
 href='/guix/tree/etc/substitutes?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>tree</a><a href='/guix/commit/etc/substitutes?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>commit</a><a href='/guix/diff/etc/substitutes?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/substitutes'>
<input type='hidden' name='id' value='79928c5e585aae10d6a15c18642bc9878f3cfa9c'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>root</a>/<a href='/guix/log/etc?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>etc</a>/<a href='/guix/log/etc/substitutes?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c'>substitutes</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/substitutes?id=79928c5e585aae10d6a15c18642bc9878f3cfa9c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-10 00:11:00 +0200'>2020-07-10</span></td><td><a href='/guix/commit/etc/substitutes?id=d283bb960f927dd5f7bb8b96bc697221e4e8ad39'>maint: Remove traces of "berlin.guixsd.org".</a><span class='msg-avail'>...<span class='msg-tooltip'>The guixsd.org domain is no longer advertised since before in 1.0.0
release in May 2019.

* etc/substitutes/berlin.guixsd.org.pub: Rename to...
* etc/substitutes/berlin.guix.gnu.org.pub: ... this.
* etc/substitutes/ci.guix.gnu.org.pub,
etc/substitutes/ci.guix.info.pub: Adjust accordingly.
* gnu/services/base.scm (%default-authorized-guix-keys): Likewise.
* guix/build/download-nar.scm (urls-for-item): Likewise.
* guix/self.scm (miscellaneous-files): Likewise.
* Makefile.am (dist_pkgdata_DATA): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>