Test py38 and py37

[ocefpaf]

We could keep testing on py36 but I don't think it is worth it.

[efiring]

We should also drop 2.7.

[ocefpaf]

We should also drop 2.7.

Done! I don't think we have any 2.7 specific code to remove though.

[efiring]

There are a bunch of "from future import" lines, but they could be removed later, unless you want to sweep them out as part of this PR.

./utide/constituent_selection.py:from __future__ import (absolute_import, division, print_function)
./utide/ellipse_params.py:from __future__ import (absolute_import, division, print_function)
./utide/confidence.py:from __future__ import (absolute_import, division, print_function)
./utide/periodogram.py:from __future__ import (absolute_import, division, print_function)
./utide/diagnostics.py:from __future__ import (absolute_import, division, print_function)
./utide/robustfit.py:from __future__ import (absolute_import, division, print_function)
./utide/__init__.py:from __future__ import (absolute_import, division, print_function)
./utide/_solve.py:from __future__ import (absolute_import, division, print_function)
./utide/_time_conversion.py:from __future__ import (absolute_import, division, print_function)
./utide/_ut_constants.py:from __future__ import (absolute_import, division, print_function)
./utide/harmonics.py:from __future__ import (absolute_import, division, print_function)
./utide/utilities.py:from __future__ import (absolute_import, division, print_function)
./utide/_reconstruct.py:from __future__ import (absolute_import, division, print_function)
./utide/data/mat2npz.py:from __future__ import (absolute_import, division, print_function)
./utide/astronomy.py:from __future__ import (absolute_import, division, print_function)
./tests/test_astron.py:from __future__ import (absolute_import, division, print_function)
./tests/test_solve.py:from __future__ import (absolute_import, division, print_function)
./tests/test_harmonics.py:from __future__ import (absolute_import, division, print_function)
./tests/test_periodogram.py:from __future__ import (absolute_import, division, print_function)
./tests/test_normalize.py:from __future__ import (absolute_import, division, print_function)
./versioneer.py:from __future__ import print_function

[ocefpaf]

Indeed. Fixing those now...

[efiring]

Thank you, @ocefpaf.