Allow disabling of python plugin

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

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",