##### 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'
```
- "File containing the CSR for the new certificate."
      - "Can be created with C(openssl req ...)."
      - "The CSR may contain multiple Subject Alternate Names, but each one
         will lead to an individual challenge that must be fulfilled for the
         CSR to be signed."
      - "I(Note): the private key used to create the CSR I(must not) be the
         account key. This is a bad idea from a security point of view, and
         the CA should not accept the CSR. The ACME server should return an
         error in this case."
    type: path
    required: true
    aliases: ['src']
  data:
    description:
      - "The data to validate ongoing challenges. This must be specified for
         the second run of the module only."
      - "The value that must be used here will be provided by a previous use
         of this module. See the examples for more details."
      - "Note that for ACME v2, only the C(order_uri) entry of C(data) will
         be used. For ACME v1, C(data) must be non-empty to indicate the
         second stage is active; all needed data will be taken from the
         CSR."
      - "I(Note): the C(data) option was marked as C(no_log) up to
         Ansible 2.5. From Ansible 2.6 on, it is no longer marked this way
         as it causes error messages to be come unusable, and C(data) does
         not contain any information which can be used without having
         access to the account key or which are not public anyway."
    type: dict
  dest:
    description: