Feature/pypy

[grigi]

Add Travis & Tox testing on PyPy, because why not.

And added supported Py versions with exception of PostgreSQL for pypy.

[coveralls]

Pull Request Test Coverage Report for Build 306

• 88 of 92 (95.65%) changed or added relevant lines in 10 files are covered.
• 309 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.02%) to 91.516%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/mysql/client.py	0	2	0.0%
tortoise/models.py	11	13	84.62%

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/asyncpg/init.py	2	100.0%
tortoise/backends/mysql/init.py	2	100.0%
tortoise/backends/asyncpg/executor.py	6	100.0%
tortoise/backends/base/client.py	8	92.42%
tortoise/backends/asyncpg/schema_generator.py	8	100.0%
tortoise/backends/mysql/schema_generator.py	13	100.0%
tortoise/backends/mysql/executor.py	21	100.0%
tortoise/backends/asyncpg/client.py	124	83.54%
tortoise/backends/mysql/client.py	125	84.3%

Totals
Change from base Build 276:	-0.02%
Covered Lines:	2091
Relevant Lines:	2225

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 316

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 313 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+6.4%) to 91.586%

---

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/asyncpg/init.py	2	100.0%
tortoise/models.py	2	94.28%
tortoise/backends/mysql/init.py	2	100.0%
tortoise/backends/asyncpg/executor.py	6	100.0%
tortoise/backends/base/client.py	8	92.42%
tortoise/backends/asyncpg/schema_generator.py	8	100.0%
tortoise/backends/mysql/schema_generator.py	13	100.0%
tortoise/backends/mysql/executor.py	21	100.0%
tortoise/backends/asyncpg/client.py	124	83.54%
tortoise/backends/mysql/client.py	127	84.3%

Totals
Change from base Build 314:	6.4%
Covered Lines:	2092
Relevant Lines:	2225

---

💛 - Coveralls

[grigi]

@Zeliboba5 Please review.

[abondar]

Great, haven't thought that it would just work without additional tuning :)