Increasing minimum python version to 3.7

[cpelley]

Originally raised in #1754 by @tjtg
I don't think there are environments in use by us or partners using <3.7 yet improver still supports python3.6 which prohibits our use of features introduced in 3.7 which are not available in 3.6

Additionally I don't think we actually test improver via the github actions with python 3.6 (we test with an environment with 3.7 environment_a, 3.8 environment_b and 3.9 conda_forge).