June fixes

[avylove]

• Fix docformatter issues failing in CI
• Gate move_right() in truncate() tests
  • This address some of the failures in #251
• Pin virtualenv for Tox
  • virtualenv recently dropped support for 2.7 and 3.6
  • This causes Tox to create a virtual environment with the pinned version and use it to create the tox environments
  • This could potentially cause another issue down the road, but probably still worth it for now

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a28a171) 95.38% compared to head (bc0d237) 95.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #255 +/- ## ======================================= Coverage 95.38% 95.38% ======================================= Files 9 9 Lines 1018 1018 Branches 215 215 ======================================= Hits 971 971 Misses 43 43 Partials 4 4 ``` | [Impacted Files](https://app.codecov.io/gh/jquast/blessed/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Quast) | Coverage Δ | | |---|---|---| | [blessed/color.py](https://app.codecov.io/gh/jquast/blessed/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Quast#diff-Ymxlc3NlZC9jb2xvci5weQ==) | `100.00% <ø> (ø)` | | | [blessed/sequences.py](https://app.codecov.io/gh/jquast/blessed/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Quast#diff-Ymxlc3NlZC9zZXF1ZW5jZXMucHk=) | `100.00% <ø> (ø)` | |

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[avylove]

Also added data_file for coverage config to support tox --parallel auto per https://github.com/nedbat/coveragepy/issues/1642

[avylove]

Tests are still failing. Apparently 2.7 was dropped from installable Python versions today. I think there are workarounds, but I'll need to investigate. We had ~54k downloads for 2.7 in the last month.