specify mistune==0.8.4 to control m2r depency verssion

[aerispaha]

Controls the mistune version (a dependency of m2r) to prevent the 'mistune' has no attribute 'BlockGrammar' error

Closes #128

[jennwuu]

@aerispaha do you need to update install_requires in setup.py?

https://github.com/aerispaha/swmmio/blob/35337dd023de58ac698414970f55ebe2161b9fab/setup.py#L34-L42

[aerispaha]

@aerispaha do you need to update install_requires in setup.py?

@jennwuu, I don't think so.

The mistune and m2r dependencies are only necessary for building the sphinx docs. I don't think a typical user would need to install those when running pip install swmmio, unless they wanted to build the docs locally. But I feel like that isn't really necessary since the docs are hosted publicly on readthedocs.

Does that make sense to you?

[jennwuu]

@aerispaha yup, that makes sense to me! Thanks for explaining.