add missing aiohttp dep

[JelleZijlstra]

Also mark 3.8 as allowed to fail for now; it will fail due to an aiohttp bug.

Fixes #690

[JelleZijlstra]

This Travis configuration just doesn't run 3.8 at all, which doesn't seem ideal. Let me try something else.

[coveralls]

Pull Request Test Coverage Report for Build 912

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 95.946%

---

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	2	97.89%

Totals
Change from base Build 910:	5.7%
Covered Lines:	2982
Relevant Lines:	3108

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 912

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 95.946%

---

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	2	97.89%

Totals
Change from base Build 910:	5.7%
Covered Lines:	2982
Relevant Lines:	3108

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 912

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 95.946%

---

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	2	97.89%

Totals
Change from base Build 910:	5.7%
Covered Lines:	2982
Relevant Lines:	3108

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 912

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 95.946%

---

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	2	97.89%

Totals
Change from base Build 910:	5.7%
Covered Lines:	2982
Relevant Lines:	3108

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 918

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 95.978%

---

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	2	97.89%

Totals
Change from base Build 910:	5.7%
Covered Lines:	2983
Relevant Lines:	3108

---

💛 - Coveralls

[asottile]

For this to run it needs to also be in the matrix

[JelleZijlstra]

Thanks for fixing!