No module named '_cffi_backend' when running somatic_variant_calling

[mbenary]

Describe the bug
When running a batch analysis for somatic variant calling, it fails with the following error message: ModuleNotFoundError: No module named '_cffi_backend'

To Reproduce
Steps to reproduce the behavior:

1. Start a snappy-pipeline with a step for somatic_variant_calling
2. Start a batch analysis using slurm.
3. Go to slurm_log for the run-step.
4. See error right after Activating environment

Expected behavior
The wrapper for somatic_variant_calling should be started and create a DAG of jobs for mutect2.

Additional context
This seems to be a combination of GATK (mutect2) and parallel jobs and / or snakemake. The step contamination in this workflow runs without error. This step also uses GATK but does not include parallelization.
The step mbcs which also relies on GATK has a dependency cffi in its wrapper, which might be necessary for the mutect2-wrapper.

[ericblanc20]

Fixed (and all dependencies were reviewed in PR #492)