Remove implicit Requests dependencies

idna, chardet, certifi, urllib3 are all not used directly by the backend.

An excerpt from pipdeptree:

arkindex==0.14.2-rc1
  - certifi [required: >=2017.4.17, installed: 2020.6.20]
  - chardet [required: >=3.0.2,<4, installed: 3.0.4]
  - idna [required: >=2.5,<3, installed: 2.10]
  - urllib3 [required: >=1.21.1,<1.26,!=1.25.1,!=1.25.0, installed: 1.22]
  - requests [required: ==2.24.0, installed: 2.24.0]
    - certifi [required: >=2017.4.17, installed: 2020.6.20]
    - chardet [required: >=3.0.2,<4, installed: 3.0.4]
    - idna [required: >=2.5,<3, installed: 2.10]
    - urllib3 [required: >=1.21.1,<1.26,!=1.25.1,!=1.25.0, installed: 1.22]