compat::getfuncargnames removes self from instance method twice by mistake

• a detailed description of the bug or suggestion
  I have a pyfuncitem which is an instance method and its signature looks like this:
  def run_test(self, testbed_obj): <== This testbed_obj is a fixture
  When getfuncargnames got called to return the argnames, it removed this testbed_obj in line 168 of compat.py, which I think is wrong, because the parameter got from line 146 of compat.py has self removed from the signature.
• output of pip list from the virtual environment you are using
  Package Version Location

astroid 1.4.9
atomicwrites 1.3.0
attrs 19.3.0
automatos-tool-library 15
avro 1.9.1
backports.functools-lru-cache 1.6.1
backports.shutil-get-terminal-size 1.0.0
backports.ssl-match-hostname 3.7.0.1
bcrypt 3.1.7
certifi 2019.11.28
cffi 1.13.2
chardet 3.0.4
colorama 0.4.3
configparser 4.0.2
confluent-kafka 1.3.0
contextlib2 0.6.0.post1
coverage 5.0.3
cryptography 2.8
decorator 4.4.1
dicttoxml 1.7.4
docker-py 1.10.6
docker-pycreds 0.4.0
enum34 1.1.6
filelock 3.0.12
flake8 3.6.0
FormEncode 1.3.1
funcsigs 1.0.2
future 0.18.2
futures 3.3.0
gitdb2 2.0.6
GitPython 2.1.14
httpretty 0.9.7
idna 2.8
importlib-metadata 1.5.0
ipaddress 1.0.23
isort 4.3.21
lazy-object-proxy 1.4.3
logutils 0.3.5
mccabe 0.6.1
mock 3.0.5
more-itertools 5.0.0
ndg-httpsclient 0.5.1
packaging 20.1
paramiko 2.7.1
pathlib2 2.3.5
pip 20.0.2
pluggy 0.13.1
py 1.8.1
pyasn1 0.4.8
pybase64 0.5.0
pycodestyle 2.4.0
pycparser 2.19
pycyclone 0.9.277
pyflakes 2.0.0
pylint 1.6.4
PyNaCl 1.3.0
pyOpenSSL 19.1.0
pyparsing 2.4.6
pysftp 0.2.9
pytest 4.6.9
pytest-html 1.22.0
pytest-metadata 1.8.0
python-nmap 0.6.1
pytz 2019.3
pyvmomi 6.7.3
PyYAML 5.3
redis 2.10.6
requests 2.22.0
requests-mock 1.6.0
requests-toolbelt 0.9.1
retrying 1.3.3
scandir 1.10.0
setuptools 44.0.0
setuptools-scm 3.2.0
six 1.14.0
smmap2 2.0.5
spur 0.3.21
suds 0.4
toml 0.10.0
tox 3.14.3
urllib3 1.23
uuid 1.30
validators 0.11.3
virtualenv 16.7.9
waiting 1.4.1
walrus 0.3.5
wcwidth 0.1.8
websocket-client 0.57.0
wheel 0.34.2
wrapt 1.11.2
xmltodict 0.12.0
zipp 1.1.0
We have some private Python packages that I don't want to list here.

• pytest and operating system versions
  pytest 4.6.9
  Windows 10
• minimal example if possible
  class MyTest(BaseTestCase):
  def run_test(self, testbed_obj):

Start in compat.py around lines 146 and 168 and trace getfuncargnames for the provided MyTest.run_test(self, testbed_obj) example. Verify the returned argument names with an instance method and add coverage showing that testbed_obj is retained rather than removed twice; done when the regression passes under pytest 4.6.9.