Econtools

- Changes to `setup.py` and `readme.md` for upload to PYPI.

Added
- New `save_mem` option for `reg` and `ivreg`. When set to True, the regression
`Results` will not save `yhat`, the `sample` flag, or `resid`. This will make
each `Results` object much smaller for regressions with large samples.
- New `check_colinear` option for `reg` and `ivreg`. When set to True,
a rank deficiency (colinearity) check will be done on the regressor matrix.
If the regressor matrix is found to be deficient (if a colinear variable is
found), the colinear columns will be calculated and printed.

Added
- Mypy typing hints.
- New function `winsorize`.
- Additional state name/abbreviation/fips crosswalk functions and lists.

Changed
- REF: Removed Python 2 compatibility (following Pandas). `econtools` now
requires Python >=3.6.
- ENH: General improvements to documentation and docstrings.
- ENH: `load_or_build` messages are now more informative.
- ENH: Updated docstrings for several functions.
- BUG: `state_name_to_fips` was not accessible for external use.

Added
- Regression `Results` objects now display formatted regression summary when
printed directly (i.e. `__repr__` method was added).
- Raw lists of state FIPS, abbreviations, and names.

Changed
- REF: The `a_name` keyword argument in regression methods has been deprecated in
favor of the new `fe_name` keyword argument.
- REF: A lot of metrics methods were refactored. This should not impact end users
but should make extending those methods a little easier.

Added
- Tests for `load_or_build`.

Changed
- REF: Tests now use `pytest` instead of `nose`.
- ENH: `outreg` now defaults to adding all regressors to the table instead of
requiring a specific list.

Initial public pseudo-release.