On Mac, shared_mutex requires minimum OS version of 10.12

[jvavrek]

Currently I get the following error when building out-of-the-box:

creating build/temp.macosx-10.9-x86_64-3.8/src
x86_64-apple-darwin13.4.0-clang -fno-strict-aliasing -Wsign-compare -Wunreachable-code -DNDEBUG -fwrapv -O3 -Wall -Wstrict-prototypes -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fPIE -fstack-protector-strong -O3 -pipe -fdebug-prefix-map=${SRC_DIR}=/usr/local/src/conda/${PKG_NAME}-${PKG_VERSION} -fdebug-prefix-map=/Users/jvavrek/opt/anaconda3/envs/myenv=/usr/local/src/conda-prefix -flto -Wl,-export_dynamic -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fPIE -fstack-protector-strong -O3 -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fPIE -fstack-protector-strong -O2 -pipe -isystem /Users/jvavrek/opt/anaconda3/envs/myenv/include -D_FORTIFY_SOURCE=2 -isystem /Users/jvavrek/opt/anaconda3/envs/myenv/include -DVERSION_INFO="0.0.3" -I/Users/jvavrek/opt/anaconda3/envs/myenv/lib/python3.8/site-packages/pybind11/include -I3rd-party/parallel-hashmap -Isrc -I/Users/jvavrek/opt/anaconda3/envs/myenv/include/python3.8 -c src/dict.cpp -o build/temp.macosx-10.9-x86_64-3.8/src/dict.o -stdlib=libc++ -mmacosx-version-min=10.7 -std=c++17 -fvisibility=hidden
clang-11: warning: -Wl,-export_dynamic: 'linker' input unused [-Wunused-command-line-argument]
In file included from src/dict.cpp:1:
In file included from 3rd-party/parallel-hashmap/parallel_hashmap/phmap.h:69:
3rd-party/parallel-hashmap/parallel_hashmap/phmap_base.h:5149:58: error: 'shared_mutex' is unavailable: introduced in macOS 10.12
    class  LockableImpl<std::shared_mutex> : public std::shared_mutex

A fix is to change

'-mmacosx-version-min=10.7'

in setup.py to

'-mmacosx-version-min=10.12'