Allow disabling of python plugin

Fixes https://github.com/pytest-dev/pytest/issues/5277.

src/_pytest/assertion/rewrite.py

@@ -60,7 +60,10 @@ class AssertionRewritingHook(object):
 
     def __init__(self, config):
         self.config = config
-        self.fnpats = config.getini("python_files")
+        try:
+            self.fnpats = config.getini("python_files")
+        except ValueError:
+            self.fnpats = ["test_*.py", "*_test.py"]
         self.session = None
         self.modules = {}
         self._rewritten_names = set()

src/_pytest/config/__init__.py

@@ -113,16 +113,16 @@ def directory_arg(path, optname):
 
 
 # Plugins that cannot be disabled via "-p no:X" currently.
-essential_plugins = (
+essential_plugins = (  # fmt: off
     "mark",
     "main",
     "runner",
-    "python",
     "fixtures",
     "helpconfig",  # Provides -p.
-)
+)  # fmt: on
 
 default_plugins = essential_plugins + (
+    "python",
     "terminal",
     "debugging",
     "unittest",

src/_pytest/fixtures.py

@@ -1075,6 +1075,15 @@ def pytestconfig(request):
     return request.config
 
 
+def pytest_addoption(parser):
+    parser.addini(
+        "usefixtures",
+        type="args",
+        default=[],
+        help="list of default fixtures to be used with this project",
+    )
+
+
 class FixtureManager(object):
     """
     pytest fixtures definitions and information is stored and managed

src/_pytest/python.py

@@ -79,15 +79,10 @@ def pytest_addoption(parser):
         default=False,
         help="show fixtures per test",
     )
-    parser.addini(
-        "usefixtures",
-        type="args",
-        default=[],
-        help="list of default fixtures to be used with this project",
-    )
     parser.addini(
         "python_files",
         type="args",
+        # NOTE: default is also used in AssertionRewritingHook.
         default=["test_*.py", "*_test.py"],
         help="glob-style file patterns for Python test module discovery",
     )