##### 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'
```
link: https://tools.ietf.org/html/draft-ietf-acme-tls-alpn-05
  - module: acme_challenge_cert_helper
    description: Helps preparing C(tls-alpn-01) challenges.
  - module: openssl_privatekey
    description: Can be used to create private keys (both for certificates and accounts).
  - module: openssl_csr
    description: Can be used to create a Certificate Signing Request (CSR).
  - module: certificate_complete_chain
    description: Allows to find the root certificate for the returned fullchain.
  - module: acme_certificate_revoke
    description: Allows to revoke certificates.
  - module: acme_account
    description: Allows to create, modify or delete an ACME account.
  - module: acme_inspect
    description: Allows to debug problems.
extends_documentation_fragment:
  - acme
options:
  account_email:
    description:
      - "The email address associated with this account."
      - "It will be used for certificate expiration warnings."
      - "Note that when C(modify_account) is not set to C(no) and you also
         used the M(acme_account) module to specify more than one contact
         for your account, this module will update your account and restrict
         it to the (at most one) contact email address specified here."
    type: str
  agreement:
    description:
      - "URI to a terms of service document you agree to when using the