pythonPackages.ofxtools: unbreak

- Remove `sqlalchemy` from `buildInputs`. This dependency was removed in
  [0.7.0]. Per the project's readme, `ofxtools` only depends on the
  standard library.

- Disable for Python versions older than 3.7, the minimum Python version
  supported by `ofxtools` starting with the [0.8.17] release.

- Fix the `checkPhase` by fetching from GitHub instead of PyPI to get a
  distribution that includes tests, overriding the home directory to
  allow tests that depend on it being writeable, and switching to
  `nosetests` to match the upstream project's test setup.

[0.7.0]: https://github.com/csingley/ofxtools/commit/8a795787d96244912fd62deda6c2c2c6e92245e8
[0.8.17]: https://github.com/csingley/ofxtools/commit/ddeda980ed5603570bff63f7a5b52c83ff713bdd