Rename `pythonpath plugin to python_path`

Fix #9636.

src/_pytest/python_path.py

@@ -0,0 +1,24 @@
+import sys
+
+import pytest
+from pytest import Config
+from pytest import Parser
+
+
+def pytest_addoption(parser: Parser) -> None:
+    parser.addini("pythonpath", type="paths", help="Add paths to sys.path", default=[])
+
+
+@pytest.hookimpl(tryfirst=True)
+def pytest_load_initial_conftests(early_config: Config) -> None:
+    # `pythonpath = a b` will set `sys.path` to `[a, b, x, y, z, ...]`
+    for path in reversed(early_config.getini("pythonpath")):
+        sys.path.insert(0, str(path))
+
+
+@pytest.hookimpl(trylast=True)
+def pytest_unconfigure(config: Config) -> None:
+    for path in config.getini("pythonpath"):
+        path_str = str(path)
+        if path_str in sys.path:
+            sys.path.remove(path_str)