Since LineageOS wasn’t mentioned as a possible solution for a while and quite a lot of Fairphone 2 users use it instead of Fairphone’s own OSes (see lineageos), I’ll leave this here …
… along with:
The LineageOS community currently supports LineageOS 16.0 on the Fairphone 2, which is Android 9 (Pie).
LineageOS is based on AOSP and in this way receives Android security updates regularly (even more regularly than Fairphone’s own OSes until Fairphone might possibly return to a monthly update schedule).
As Pokemon GO relies on Google services, which LineageOS doesn’t include by default, you would additionally need to install Open GApps (ARM - 9.0 - package of your choice, “pico” is the minimum to get the Play Store).
These are the real Google Apps and services, the “Open” only refers to the packaging of different variants and the easy installation as a ZIP file via the TWRP recovery program.
To be on the safe side … LineageOS is Android, but not an Android OS certified by Google (in contrast to Fairphone’s own OSes).
Google lets you register devices running uncertified Android OSes for the legitimate use of Google services at https://www.google.com/android/uncertified/ .