Aemstro is a set of tools to assemble and disassemble shader code for the PICA200 GPU used in the Nintendo 3DS. It was the first of such toolkits to be released, and as such is lacking in functionality and less user friendly compared to later tools such as Nihstro and picasso. As such, it is not recommended to use this toolset unless you have very specific use cases.
In order to install this library, simply download the aemstro python files from GitHub and put them in the msys folder of your devkitpro install. On Linux/Mac, you can place the files anywhere you want, and have to add an environment variable "AEMSTRO" with the location of where you placed them.
This library also requires you to have a version of Python 3 installed. If you have a version of Python 2 installed, you can install Python 3 simultaneously (on windows this may require additional configuration).