Support arrow >= 1.0

[gavinhodge]

There are some breaking changes in the arrow 1.0 release. I noticed when using toyplot.locator.Timestamp, as the ticks() function will crash.

Toyplot doesn't pin the arrow dependency, so pip will install the latest version by default.

Notably, timestamp has been renamed to int_timestamp, and replaced by a function timestamp(). So the various tick generators need to either reference int_timestamp or call timestamp().

I'm not sure which is better for backwards compatibility.

[gavinhodge]

Arrow v1.0 migration guide:
arrow-py/arrow#832

[tshead2]

@gavinhodge - many thanks for this, will work on a fix right away.

Cheers,
Tim

[jdtuck]

@tshead2 Do we have a fix for this? since it is not pinned and arrow has gone to 1.2 makes it a pain to just install it.

[tshead2]

Fixed in e0cf9da.