Fix file encoding for some loads.

We've actually been pretty good at getting this right in most of the
code base, but we've missed it in a few places. Python defaults to the
encoding of the current locale unless otherwise specified, and for a US
English Windows install that's cp1252, not UTF-8. I'm not brave enough
to change the locale at startup because I don't know how that might
affect CJK encoding users (or for that matter, non-Latin derived
alphabet UTF-8 variants).

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/2786.

game/campaignloader/campaign.py

@@ -57,7 +57,7 @@ class Campaign:
 
     @classmethod
     def from_file(cls, path: Path) -> Campaign:
-        with path.open() as campaign_file:
+        with path.open(encoding="utf-8") as campaign_file:
             data = yaml.safe_load(campaign_file)
 
         sanitized_theater = data["theater"].replace(" ", "")