Fixed GraphicsContextBase linestyle getter

[Kojoley]

Closes #3354

[tacaswell]

For reference, this has been like this since e34a333 and has no previous history.

We also do not appear to ever use it internally. All of our backends use get_dashes instead which (mostly) ensures that the dash pattern is the same across all backends (and enables us to scale the pattern with line width).

This needs an API changes note (there may be third-party backends working around this), but given that the current API is obviously problematic (and no one has complained) I do not think it needs a deprecation cycle.

[Kojoley]

GraphicsContextBase.get_linestyle is not used anywhere in mpl code.
GraphicsContextBase._linestyle is used only in GraphicsContextBase.copy_properties and it sets the variable DIRECTLY.
GraphicsContextBase.set_linestyle is used only in one place and overridden only in wx backend.
There is a TODO entry in pdf backend about _linestyle from 2006 year and I am not sure it makes any sense now.

So I assume it is safe to fix this without deprecation (or even to remove the property).

[anntzer]

If we're going to break backcompat, let's just remove the property?

[tacaswell]

power cycled to run CI with fixes from master branch to other tests.

[codecov-io]

Current coverage is 62.52% (diff: 100%)

Merging #7730 into master will increase coverage by 0.40%

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[NelleV]

LGTM 👍
Thanks @Kojoley