Add trove classifiers for all supported Python versions
Add trove classifiers for all supported Python versions

I frequently use the PyPI trove classifiers to check if a 3rd party
package is usable by my project. Documenting all supported versions
makes this easier.

Disassem/urllib3,Disassem/urllib3,sigmavirus24/urllib3,urllib3/urllib3,sigmavirus24/urllib3,urllib3/urllib3