Tests fail while setting up GPG

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running `python -m pytest` leads to:
```
============================= test session starts ==============================
platform linux -- Python 3.8.2, pytest-5.4.2, py-1.8.1, pluggy-0.13.1
rootdir: /build/python-requests-gpgauthlib/src/requests_gpgauthlib-0.1.2
plugins: requests-mock-1.8.0
collected 29 items

tests/unit/test_gpgauthsession.py .EEEEEEEEEEEEEEEEE [ 62%]
tests/unit/test_utils.py ......FFFFF [100%]

==================================== ERRORS ====================================
_ ERROR at setup of TestGPGAuthSession.test_gpgauth_version_is_supported_not_in_absence_of_headers _

self =

def setup_class(self):
# Setup a server
self.server_gnupg_home = get_temporary_workdir()
> self.server_gpg = create_gpg(self.server_gnupg_home.name)

tests/unit/test_gpgauthsession.py:49:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
requests_gpgauthlib/utils.py:43: in create_gpg
gpg = GPG(gnupghome=os.path.join(workdir, '.gnupg'))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = , gpgbinary = 'gpg'
gnupghome = '/tmp/requests_gpgauthlib-vx9ib7ms/.gnupg', verbose = False
use_agent = False, keyring = None, options = None, secret_keyring = None

def __init__(self, gpgbinary='gpg', gnupghome=None, verbose=False,
use_agent=False, keyring=None, options=None,
secret_keyring=None):
"""Initialize a GPG process wrapper. Options are:

gpgbinary -- full pathname for GPG binary.

gnupghome -- full pathname to where we can find the public and
private keyrings. Default is whatever gpg defaults to.
keyring -- name of alternative keyring file to use, or list of such
keyrings. If specified, the default keyring is not used.
options =-- a list of additional options to pass to the GPG binary.
secret_keyring -- name of alternative secret keyring file to use, or
list of such keyrings.
"""
self.gpgbinary = gpgbinary
self.gnupghome = gnupghome
# issue 112: fail if the specified value isn't a directory
if gnupghome and not os.path.isdir(gnupghome):
> raise ValueError('gnupghome should be a directory (it isn\'t): %s' % gnupghome)
E ValueError: gnupghome should be a directory (it isn't): /tmp/requests_gpgauthlib-vx9ib7ms/.gnupg

/usr/lib/python3.8/site-packages/gnupg.py:827: ValueError
_ ERROR at setup of TestGPGAuthSession.test_gpgauth_version_is_supported_not_for_wrong_versions _

self =

def setup_class(self):
# Setup a server
self.server_gnupg_home = get_temporary_workdir()
> self.server_gpg = create_gpg(self.server_gnupg_home.name)
```

Start with create_gpg in requests_gpgauthlib/utils.py and the setup_class path in tests/unit/test_gpgauthsession.py, then run python -m pytest to reproduce the failure. Check how the temporary GPG home is prepared before initialization; done means GPG setup completes and the existing test suite passes.

Written by the indexing model from the issue text.