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/sqlite.py

@@ -48,4 +48,5 @@ def sqlite_copy_and_open(db: PathIsh) -> sqlite3.Connection:
         with sqlite3.connect(str(tdir / dp.name)) as conn:
             from .compat import sqlite_backup
             sqlite_backup(source=conn, dest=dest)
+        conn.close()
     return dest