core/ci: fix windows-specific issues

- use portable separators
- paths should be prepended with r' (so backwards slash isn't treated as escaping)
- sqlite connections should be closed (otherwise windows fails to remove the underlying db file)
- workaround for emojis via PYTHONUTF8=1 test for now
- make ZipPath portable
- properly use tox python environment everywhere

  this was causing issues on Windows
  e.g.
      WARNING: test command found but not installed in testenv
        cmd: C:\hostedtoolcache\windows\Python\3.9.12\x64\python3.EXE

my/core/kompress.py

@@ -203,7 +203,9 @@ class ZipPath(ZipPathBase):
     def stat(self) -> os.stat_result:
         # NOTE: zip datetimes have no notion of time zone, usually they just keep local time?
         # see https://en.wikipedia.org/wiki/ZIP_(file_format)#Structure
-        dt = datetime(*self.root.getinfo(str(self.subpath)).date_time)
+        # note: seems that zip always uses forward slash, regardless OS?
+        zip_subpath = '/'.join(self.subpath.parts)
+        dt = datetime(*self.root.getinfo(zip_subpath).date_time)
         ts = int(dt.timestamp())
         params = dict(
             st_mode=0,