Pin irc library version to 20.4.0

this version was in the recent 0.21.0 release and worked fine

the irc library is broken (for us) since jaraco/irc@c95c063
the reason is that it uses importlib.resources.files() to import codes
and this function is buggy -- it doesn't work with freezed code
(see also python/cpython#121735. wimglenn/resources-example#6)
and (what a coincidence!) broken implementation of files() function was written
by the same person -- see python/cpython#100598

requirements.txt

@@ -1,7 +1,7 @@
 cx_Freeze
 idna
 ipaddress
-irc
+irc==20.4.0
 jinja2
 pathlib
 pydantic