Moving back to pyflakes with flymake for Python.

This largely reverts 9fa92847b20e1d562e1f4e191bd9faf72e6e9945.

Flycheck is an excellent piece of software for other
languages. However, for Python I encountered the following problems:

1. PEP8 issues are errors, but pyflakes issues (which can actually
represent bugs) are warnings.
https://github.com/flycheck/flycheck/issues/158

2. I'm not interested in PEP8 style issues, the rules are often broken
on code that has been written by different people. In this case, I
just want to find bugs, not change someone else's style. This forces
me to write monstrosities like this in my ~/.config/flake8:

ignore = W191,W291,W292,W293,W391,W602,E41,E101,E121,E122,E123,E124,E125,E126,E127,E128,E203,E221,E225,E226,E241,E261,E231,E251,E271,E272,E301,E302,E303,E701,E702,E703

(I was unable to find a way to split this onto separate lines.)

3. Flycheck no longer supports pyflakes as a standalone
tool. https://github.com/flycheck/flycheck/issues/115

Known bugs:
* Only one flymake error is shown in the minibuffer at once.

Future work:
* I'd like to move back to storing third-party scripts in this git
  repo, to minimise the dependencies and shorten the the setup time
  on a new system.