labm8/py/test: Move global conftest into //labm8/py/internal.

This moves the conftest.py file at the root of the WORKSPACE into a
module in //labm8/py/internal that is explicitly loaded as a plugin by
//labm8/py:test.

This is part of the ongoing process to eliminate conftest.py files
from the repo and to enable --noconftest arg in pytest.

github.com/ChrisCummins/phd/issues/70

Signed-off-by: format 2020.01.16 <github.com/ChrisCummins/format>

[Exported from 0136b42163265816ce8087a73cfcd27a76647ee2]

BUILD

@@ -75,18 +75,6 @@ python_grpc_library(
     deps = ["//:config_pb"],
 )
 
-py_library(
-    name = "conftest",
-    testonly = 1,
-    srcs = ["conftest.py"],
-    visibility = ["//visibility:public"],
-    deps = [
-        "//:build_info",
-        "//labm8/py:app",
-        "//third_party/py/pytest",
-    ],
-)
-
 # Golang.
 # Gazelle directive:
 # gazelle:prefix github.com/ChrisCummins/phd