Allow tox version to be specified

[samaloney]

Allow a version to passed to the tox install command:

azure-pipelines-templates/run-tox-env.yml

Lines 188 to 191 in 7f70d92

	- ${{ if variables.docker_image }}:
	- script: docker exec -i -w /project ${{ variables.docker_name }} ${{ variables.setarch }} ${{ variables.docker_python_exec }} -m pip install --upgrade tox ${{ variables.toxdeps }}
	displayName: Install tox

azure-pipelines-templates/run-tox-env.yml

Lines 196 to 199 in 7f70d92

	- ${{ if not(variables.docker_image) }}:
	- script: python -m pip install --upgrade tox ${{ variables.toxdeps }}
	displayName: Install tox

[samaloney]

So as a workaround this can hacked in toxdeps which works but isn't very transparent e.g. toxdeps: "tox<4". #85 tried to add a toxverspec variable is maybe a little more clear.