pep8 for backend_pdf.py
remove backend_pdf from pep8 test exclude list
minor code standard changes
minor code formatting
Merge pull request #2562 from esc/minor_recipe_tweaks

Just some small tweaks to the recipes
more informative exceptions for empty/not-existing images in
compare_images()
add titleweight rc param
actually read the new rcParam
reference rcparam in docstring for pyplot interface
fix pep8 errors.
Merge pull request #2462 from pelson/path_effects_update

Path effects update
adds rcParam `axes.formatter.useoffset`

Addresses issue #2400

added test for axes.formatter.useoffset rcparam
MultiCursor: make events connected during __init__ accessible (for later removal)
fix operation order error in _spectral_helper and implement more tests to catch these sorts of problems in the future
fix initialization of AnnotationBbox where None values of xybox and boxcoords parameters are not correctly handled.
AffineBase.__eq__ should not raise an exception when the other does not have "is_affine" attribute.
Merge pull request #2581 from leejjoon/affine_base_eq

AffineBase.__eq__ should not raise an exception when the other does not ...
Merge pull request #2495 from vlsd/py3gscheck

fixed an enconding bug when checking for gs version
Merge pull request #2401 from tacaswell/useoffset_rcparam

adds rcParam `axes.formatter.useoffset`
make event connector ids private