Add requirements.txt to automated build

[FreddieAkeroyd]

This allows specification of items needed for python venv on ISIS doc build server

See #899

It is possible to use a requirements.txt from a conda environment.yml

name: test-env
dependencies:
  - python>=3.5
  - anaconda
  - pip
  - pip:
    - -r file:requirements.txt

[FreddieAkeroyd]

have reduced requirements.txt to same 4 entries as environment.yml and it works fine

[prjemian]

Lots of items in the requirements, many of which are probably required by a smaller number of packages we name here. pip will examine the requirements of each package named in this file. This list may need to be pruned later as the packages we actually need update their requirements. If it works now, we can defer the pruning actions to a later issue.

[FreddieAkeroyd]

I pruned requirements.txt and it still seems to work on our local build server

[prjemian]

LGTM