Drop support for Python 2.7

[thibaudcolas]

There is no plans to do this immediately, just opening this early so people interested in this project have a fair heads-up.

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I'm looking into Python versions support of the exporter as part of #101. Although that issue isn't anything like a pressing feature / bug to address, I think it's important to be clear that the exporter isn't going to support old versions of Python forever.

Python 2.7 will officially stop being supported as a language on January 1st 2020 (PEP-373), 8 months from now (https://pythonclock.org/). The exporter should also drop Python 2.7 support then, if not before, like many other projects (https://python3statement.org/). The exporter is relatively stable, so there is no need to do this sooner than needed, either in 2020 to align with other projects in the Python ecosystem, or earlier if there is important work done on the exporter and it feels like Python 2.7 support is a hindrance.

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There are many people relying on this project and using Python 2.7 at the moment, here are the pip installs over the last 30 days:

| python_version | download_count |
| -------------- | -------------- |
| 3.6            |         18,249 |
| 2.7            |         11,815 |
| 3.7            |          8,106 |
| 3.5            |          3,112 |
| 3.4            |            384 |
| 3.3            |              4 |
| 3.8            |              3 |
| 2.6            |              1 |
| Total          |         41,674 |

If you're one of them, it would be good to hear from you! Please be assured that if upgrading Python versions isn't an option for you, older versions of the exporter will still be available and keep on working. Considering how stable this project is, they'll also most likely keep on being relevant.