Support dtype arg of spaces.Box

[toslunar]

gym 0.9.6 added dtype arg to spaces.Box.
The PR supports the newer interface in a backward-compatible way.
The fix in chainerrl.envs removes the warnings WARN: gym.spaces.Box autodetected dtype as < ... >. Please provide explicit dtype..

[toslunar]

I run in Python 3 the test pytest -m "not slow and not gpu" tests -x with gym versions:

• 0.7.3 (the current requirement of ChainerRL),
• 0.9.5,
• 0.9.6.

[toslunar]

Passed the tests

• ./test_examples -1,
• ./test_examples 0, and
• pytest -m "not slow"

with the versions above (i.e. I tested 3×3). Ready to be reviewed.

[muupan]

LGTM