BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
ENH better error message when wrong cmap name.

Now prints all possible choices
Re-enabled PEP8 test, closing #2443.
add failing test: savefig(StringIO(), format="ps")
Merge pull request #2428 from cpelley/symlognorm_fix

BUG: Fixed object type missmatch in SymLogNorm
BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
Merge remote-tracking branch 'upstream/v1.3.x'
Merge pull request #2491 from pelson/pep8_re-enable

Re-enabled PEP8 test, closing #2443.
Merge pull request #2485 from NelleV/error_message

ENH better error message when wrong cmap name.
add new ps backend test file in `__init__.py`
Fix writing to all manner of StringIO-like objects in PS backend.
Merge pull request #2512 from mdboom/ps_StringIO

Fix saving to in-memory file-like objects in Postscript backend
Add easy style sheet selection.
Add rc_params_in_file to return partially-filled RcParams

This allows style sheets to update the current settings instead of overwriting them.
Rename style files to `*.style`
Allow style.use to open URLs
Remove pyplot import
Add style context manager and tests
Change style extension to *.mplstyle
Got a little crazy with the whitespace