Add cffi to runtime requirements

[maxnoe]

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[maxnoe]

Using pip check complained that zstandard is missing the cffi dependency after installing from conda forge, this should fix it.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipe) and found some lint.

Here's what I've got...

For recipe:

• Failed to even lint the recipe, probably because of a conda-smithy bug :cry:. This likely indicates a problem in your meta.yaml, though. To get a traceback to help figure out what's going on, install conda-smithy and run conda smithy recipe-lint . from the recipe directory.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[rmax]

I could reproduce it:

(base) root@16b5f0f237b7:/# conda create -n testenv2 python=3.8 zstandard
Collecting package metadata (current_repodata.json): done
Solving environment: done

## Package Plan ##

  environment location: /opt/conda/envs/testenv2

  added / updated specs:
    - python=3.8
    - zstandard

The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    python-3.8.6               |h852b56e_0_cpython        22.1 MB  conda-forge
    zstandard-0.14.0           |   py38h950e882_2         582 KB  conda-forge
    ------------------------------------------------------------
                                           Total:        22.7 MB

The following NEW packages will be INSTALLED:

  _libgcc_mutex      conda-forge/linux-64::_libgcc_mutex-0.1-conda_forge
  _openmp_mutex      conda-forge/linux-64::_openmp_mutex-4.5-1_gnu
  ca-certificates    conda-forge/linux-64::ca-certificates-2020.6.20-hecda079_0
  certifi            conda-forge/linux-64::certifi-2020.6.20-py38h924ce5b_2
  ld_impl_linux-64   conda-forge/linux-64::ld_impl_linux-64-2.35-h769bd43_9
  libffi             conda-forge/linux-64::libffi-3.2.1-he1b5a44_1007
  libgcc-ng          conda-forge/linux-64::libgcc-ng-9.3.0-h5dbcf3e_17
  libgomp            conda-forge/linux-64::libgomp-9.3.0-h5dbcf3e_17
  libstdcxx-ng       conda-forge/linux-64::libstdcxx-ng-9.3.0-h2ae2ef3_17
  ncurses            conda-forge/linux-64::ncurses-6.2-he1b5a44_2
  openssl            conda-forge/linux-64::openssl-1.1.1h-h516909a_0
  pip                conda-forge/noarch::pip-20.2.4-py_0
  python             conda-forge/linux-64::python-3.8.6-h852b56e_0_cpython
  python_abi         conda-forge/linux-64::python_abi-3.8-1_cp38
  readline           conda-forge/linux-64::readline-8.0-he28a2e2_2
  setuptools         conda-forge/linux-64::setuptools-49.6.0-py38h924ce5b_2
  sqlite             conda-forge/linux-64::sqlite-3.33.0-h4cf870e_1
  tk                 conda-forge/linux-64::tk-8.6.10-hed695b0_1
  wheel              conda-forge/noarch::wheel-0.35.1-pyh9f0ad1d_0
  xz                 conda-forge/linux-64::xz-5.2.5-h516909a_1
  zlib               conda-forge/linux-64::zlib-1.2.11-h516909a_1010
  zstandard          conda-forge/linux-64::zstandard-0.14.0-py38h950e882_2

Proceed ([y]/n)? y

Downloading and Extracting Packages
python-3.8.6         | 22.1 MB   | ####################################################################################################################################### | 100%
zstandard-0.14.0     | 582 KB    | ####################################################################################################################################### | 100%
Preparing transaction: done
Verifying transaction: done
Executing transaction: done
#
# To activate this environment, use
#
#     $ conda activate testenv2
#
# To deactivate an active environment, use
#
#     $ conda deactivate

(base) root@16b5f0f237b7:/# conda activate testenv2
(testenv2) root@16b5f0f237b7:/# pip check
zstandard 0.14.0 requires cffi, which is not installed.

[github-actions[bot]]

Hi! This is the friendly conda-forge automerge bot!

I considered the following status checks when analyzing this PR:

• linter: passed
• drone: passed
• travis: passed
• azure: passed

Thus the PR was passing and merged! Have a great day!