Re-enable PyPy build

[Luthaf]

As part of the update to 0.10 in #15, I had to disable the pypy build since it was failing to resolve dependencies.

See https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=274769&view=logs&jobId=9a864fd9-6c8f-52ca-79ce-2aa6dca1a1de&j=9a864fd9-6c8f-52ca-79ce-2aa6dca1a1de&t=10fc5aa2-324e-5982-4c88-6b31fcab16b3 and https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=274769&view=logs&jobId=15f8dc28-9f6f-5e3a-5b3c-2b169071e5be&j=15f8dc28-9f6f-5e3a-5b3c-2b169071e5be&t=63fa899e-c160-5185-75c4-c5fd30c90374 for the failed logs, I also copy-paste the relevant bit below.

Output in format: Requested package -> Available versions

Package expat conflicts for:
scikit-build -> cmake -> expat[version='2.2.*|>=2.2.10,<2.3.0a0|>=2.2.9,<2.3.0a0|>=2.2.5,<2.3.0a0|>=2.2.10,<3.0a0|>=2.2.9,<3.0a0|>=2.2.6,<3.0a0|>=2.2.5,<3.0a0']
numpy=1.18 -> pypy3.6[version='>=7.3.1'] -> expat[version='>=2.2.9,<2.3.0a0']
python=3.6[build=*_73_pypy] -> pypy3.6=7.3.3 -> expat[version='>=2.2.9,<2.3.0a0']

Package _openmp_mutex conflicts for:
scikit-build -> libgcc-ng[version='>=9.3.0'] -> _openmp_mutex[version='>=4.5']
numpy=1.18 -> libgcc-ng[version='>=7.5.0'] -> _openmp_mutex[version='*|>=4.5',build=*_llvm]
chemfiles-lib=0.10 -> libgcc-ng[version='>=9.3.0'] -> _openmp_mutex[version='>=4.5']

Package gdbm conflicts for:
python=3.6[build=*_73_pypy] -> pypy3.6=7.3.3 -> gdbm[version='>=1.18,<1.19.0a0']
numpy=1.18 -> pypy3.6[version='>=7.3.1'] -> gdbm[version='>=1.18,<1.19.0a0']

Package libstdcxx-ng conflicts for:
numpy=1.18 -> python[version='>=3.6,<3.7.0a0'] -> libstdcxx-ng[version='>=4.9|>=7.3.0|>=7.5.0|>=9.3.0|>=7.2.0']
wheel -> python -> libstdcxx-ng[version='>=4.9|>=7.3.0|>=7.5.0|>=9.3.0|>=7.2.0']
pip -> python[version='>=3.6'] -> libstdcxx-ng[version='>=4.9|>=7.3.0|>=7.5.0|>=9.3.0|>=7.2.0']
scikit-build -> cmake -> libstdcxx-ng[version='>=7.2.0']
scikit-build -> libstdcxx-ng[version='>=4.9|>=7.3.0|>=7.5.0|>=9.3.0']
Package curl conflicts for:
chemfiles-lib=0.10 -> libnetcdf[version='>=4.7.4'] -> curl[version='>=7.64.1,<8.0a0|>=7.69.1,<8.0a0|>=7.71.1,<8.0a0']
scikit-build -> cmake -> curl[version='>=7.44.0,<8|>=7.59.0,<8.0a0|>=7.61.1,<8.0a0']
Note that strict channel priority may have removed packages required for satisfiability.

I don't know many users of this package which use PyPy since this depends on numpy which had bad performances under pypy last time I checked. If you want/need to use chemfiles with pypy, please let me know here!

[Luthaf]

fixed in #22