Error running "poetry install" as per Contributing documentation

issue.md

With a clean pelican repo run the following commands, as per the contributing documentation:

$ mkdir ~/virtualenvs
$ python3 -m venv ~/virtualenvs/pelican
$ source ~/virtualenvs/pelican/bin/activate

(pelican) $ pip install -e ~/projects/pelican invoke
(pelican) $ invoke setup
...
[This is the output of "poetry install"]
Installing dependencies from lock file
Warning: The lock file is not up to date with the latest changes in pyproject.toml.
You may be getting outdated dependencies. Run update to update them.

[SolverProblemError]                                                            
Because pelican depends on invoke (^1.3) which doesn't match any versions,
version solving failed.     

install [--no-dev] [--dry-run] [-E|--extras EXTRAS] [--develop DEVELOP]

If I try to update poetry.lock by running poetry update, I got this error:

Updating dependencies
Resolving dependencies... (2.8s)
                                                                                    
[SolverProblemError]                                                 
The current project must support the following Python versions: ~2.7 || ^3.5      
Because pytest-emoji (0.2.0) requires Python >=3.4                                
 and no versions of pytest-emoji match >0.2.0,<0.3.0, pytest-emoji is forbidden.  
So, because pelican depends on pytest-emoji (^0.2.0), version solving failed.     
                                                                                    
update [--no-dev] [--dry-run] [--lock] [--] [<packages>]...