Drop Py27 and Win32; add Py38

[dwhswenson]

Currently our Travis script has a Py27/MDTraj dev test job. However, MDTraj has made changes that are incompatible with Py27. We'd already discussed dropping Py27 and Win32 from tests (#69 (comment)), so I'm doing it now.

While I'm at it, I'm including Py38 in our test matrix (overdue!). Note that the next release will be the last one to guarantee support Python 3.6 (by NEP29 we could already drop 3.6, but I'm going to take a slightly more conservative approach.)

[dwhswenson]

Going to close/open to see if I can get travis to trigger

[dwhswenson]

Summary of what's in here:

• We no longer test against Python 2 at all
• We no longer test against Win32 at all
• Travis tests Python 3.6, 3.7, 3.8, and considers 3.8 the "canonical" version (and 3.8 is tested against MDTraj current master as well as MDTraj release; others only tested against MDTraj release)
• AppVeyor tests Python 3.6, 3.7. (It seems that no one is using AppVeyor's Miniconda38 yet; AppVeyor lists it as deployed, but I couldn't find any evidence of widespread use)

Also, I migrated us to travis-ci.com (from the .org). There's a general (but very gentle) push from the Travis folks to go that direction.

As a thought for the future, it might make sense for testing against MDTraj master to be skipped on the stable branch. But that's for a future PR.

[sroet]

1 question, 1 proposed change (feel free to ignore), and some commented out leftover code, looks good to me otherwise

[sroet]

LGTM, feel free to merge