Drop python 3.7 builds

conda/conda_build_config.yaml

-macos_min_version:
-  - 10.9
-macos_machine:
-  - x86_64-apple-darwin13.4.0
-MACOSX_DEPLOYMENT_TARGET:
-  - 10.9
-CONDA_BUILD_SYSROOT:            # [osx]
-  - /opt/MacOSX10.9.sdk         # [osx]
-# This helps CMAKE find the sysroot. See
-# https://cmake.org/cmake/help/v3.11/variable/CMAKE_OSX_SYSROOT.html
-SDKROOT:                        # [osx]
-  - /opt/MacOSX10.9.sdk         # [osx]
-# makes autotools verbose
-VERBOSE_AT:
-  - V=1
-# makes cmake verbose
-VERBOSE_CM:
-  - VERBOSE=1
-
 ## the dependencies that we build against multiple versions
 python:
-  - 3.7
   - 3.8
   - 3.9