test: Root level conftest

________________________ ERROR collecting test session _________________________
Defining 'pytest_plugins' in a non-top-level conftest is no longer supported:
It affects the entire test suite instead of just below the conftest as expected.
  /home/runner/work/libtmux/libtmux/src/libtmux/conftest.py
Please move it to a top level conftest file at the rootdir:
  /home/runner/work/libtmux/libtmux
For more information, visit:
  https://docs.pytest.org/en/stable/deprecations.html#pytest-plugins-in-non-top-level-conftest-files

Author: tony

conftest.py

@@ -0,0 +1 @@
+pytest_plugins = ["pytester"]

src/libtmux/conftest.py

@@ -11,8 +11,6 @@
 if t.TYPE_CHECKING:
     from libtmux.session import Session
 
-pytest_plugins = ["pytester"]
-
 
 @pytest.fixture(autouse=True)
 def add_doctest_fixtures(