Fix PyVCF3 requirement

[gpanouts]

If a user has a python virtual environment with PyVCF3==1.0.0 and then install the varsome api client, the PyVCF3 will not be updated since it meets the project’s requirement which is PyVCF3>=1.0.0. However this leads to an error:

$ varsome_api_annotate_vcf.py -g hg19 -k ${API_KEY} -i plain_variants.vcf -o out.vcf -p add-all-data=1
Traceback (most recent call last):
  File "/tmp_varsome_api/test_varsome_102423/bin/varsome_api_annotate_vcf.py", line 19, in <module>
    from varsome_api.vcf import VCFAnnotator
  File "/tmp_varsome_api/test_varsome_102423/lib/python3.7/site-packages/varsome_api/vcf.py", line 20, in <module>
    from vcf.parser import _Info, _encode_type
ImportError: cannot import name '_encode_type' from 'vcf.parser' (/tmp_varsome_api/test_varsome_102423/lib/python3.7/site-packages/vcf/parser.py)

We need to update the requirement to PyVCF3>=1.0.1 to resolve this.