Add flake8 dependencies to requirements-dev.txt file

[dorothykiz1]

closes #1074

[datapythonista]

Not sure what's the reasoning here, but we want just the latest version of flake8 (the latest at every time, not the latest now), and no other dependencies. If flake8 needs those other packages, they'll be installed automatically.

[dorothykiz1]

Thanks @datapythonista

[datapythonista]

Small thing, but better to not introduce arbitrary blank lines, and keep our changes to exactly what we want.

[datapythonista]

closes [#1074 ]

The squared brackets around the issue will prevent the issue from being automatically closed when this PR is merged. Not sure what are the exact idioms that GitHub accepts, but Closes #1074 surely work (I think closes #1074, Close #1074... are also fine). I think GitHub shows you if you hover after that text, whether this is really going to close the issue or not.

[dorothykiz1]

Thank you @datapythonista i didn't know the hovering gives you a clue of what closes will do.

[datapythonista]

Thanks @dorothykiz1