bug: upgrade pygrib version from 2.1.6 to 2.1.8

[mackenzie-grimes-noaa]

Linear Issue

IDSSE-1616

Changes

• Bump pygrib Python library version from 2.1.6 to 2.1.8. Changelog

Explanation

Only in the dev environment (not on my local macos machine), DAS was seeing constant errors when it attempted to read some GRIB variable with pygrib (see DAS shrink_grib code):

RuntimeError: Unable to preserve variables from GRIB file due to exception: 
  (<class 'TypeError'>) an integer is required

The sole maintainer of pygrib has published a new minor version since we last attempted. Maybe there was some mysterious incompatibility with another library we were using that has since been fixed? Or moving from 2.1.4 to 2.1.6 introduced the error?