Move benchmarks to > 3.7

[hmacdope]

Expected behavior

Since support has been dropped for 2.7 and 3.6 should bump the benchmarks from the current 2.7 python up to whatever version is appropriate.

This is mainly a reminder to change the value in asv.conf.json from:

    "pythons": ["2.7"],

to whatever version is appropriate once bench environment is setup.

Linked to MDAnalysis/benchmarks#11

Actual behavior

tests fail to run unless pythons=2.7 is commented out.

Code to reproduce the behavior

running the benchmarks with asv run or asv continuous

[orbeckst]

See also MDAnalysis/benchmarks#11

[orbeckst]

MDAnalysis/benchmarks#11 has been closed and the benchmarks are running again.

[orbeckst]

oops, have to fix this in MDA ...