##### SUMMARY
acme_certificate fails on python 2.7
```
  File "/tmp/ansible_acme_certificate_payload_E_vyop/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 762, in f
AttributeError: 'list' object has no attribute 'clear'
```

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
acme_certificate

##### ANSIBLE VERSION
```
ansible 2.9.0
```

##### OS / ENVIRONMENT
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic
Python 2.7.15+

##### STEPS TO REPRODUCE
Issue a certificate with acme_certificate.

```yaml
  - name: "Request certificate for {{ CERT.CN }}"
    acme_certificate:
      account_key: /etc/ssl/private/account.key
      csr: "/etc/ssl/csr/{{ CERT.CN }}.csr"
      dest: "/etc/ssl/certs/{{ CERT.CN }}.crt"
      acme_directory: "https://acme-v01.api.letsencrypt.org/directory"
      agreement: "https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf"
      data: "{{ letsencrypt_challenge }}"
      challenge: "dns-01"
      force: "{{ csr.changed }}"
      remaining_days: "10"
```

##### EXPECTED RESULTS
Successful issued certificate.


##### ACTUAL RESULTS
```paste below
The full traceback is:
Traceback (most recent call last):
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 102, in <module>
    _ansiballz_main()
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 94, in _ansiballz_main
    invoke_module(zipped_mod, temp_path, ANSIBALLZ_PARAMS)
  File "/root/.ansible/tmp/ansible-tmp-1573040538.74-14433469763146/AnsiballZ_acme_certificate.py", line 40, in invoke_module
    runpy.run_module(mod_name='ansible.modules.crypto.acme.acme_certificate', init_globals=None, run_name='__main__', alter_sys=False)
  File "/usr/lib/python2.7/runpy.py", line 192, in run_module
    fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 1090, in <module>
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 1061, in main
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 921, in get_certificate
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 766, in _new_cert_v1
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/module_utils/acme.py", line 943, in process_links
  File "/tmp/ansible_acme_certificate_payload_uUsYVr/ansible_acme_certificate_payload.zip/ansible/modules/crypto/acme/acme_certificate.py", line 762, in f
AttributeError: 'list' object has no attribute 'clear'
```
import cryptography.hazmat.backends
    import cryptography.x509
except ImportError:
    CRYPTOGRAPHY_IMP_ERR = traceback.format_exc()
    CRYPTOGRAPHY_FOUND = False
else:
    CRYPTOGRAPHY_FOUND = True
def get_cert_days(module, cert_file):
    '''
    Return the days the certificate in cert_file remains valid and -1
    if the file was not found. If cert_file contains more than one
    certificate, only the first one will be considered.
    '''
    if HAS_CURRENT_CRYPTOGRAPHY:
        return cryptography_get_cert_days(module, cert_file)
    if not os.path.exists(cert_file):
        return -1
    openssl_bin = module.get_bin_path('openssl', True)
    openssl_cert_cmd = [openssl_bin, "x509", "-in", cert_file, "-noout", "-text"]
    dummy, out, dummy = module.run_command(openssl_cert_cmd, check_rc=True, encoding=None)
    try:
        not_after_str = re.search(r"\s+Not After\s*:\s+(.*)", out.decode('utf8')).group(1)
        not_after = datetime.fromtimestamp(time.mktime(time.strptime(not_after_str, '%b %d %H:%M:%S %Y %Z')))
    except AttributeError:
        raise ModuleFailException("No 'Not after' date found in {0}".format(cert_file))
    except ValueError:
        raise ModuleFailException("Failed to parse 'Not after' date of {0}".format(cert_file))
    now = datetime.utcnow()
    return (not_after - now).days
class ACMEClient(object):