Bump asyncpg from 0.19.0 to 0.20.0

[dependabot-preview]

Bumps asyncpg from 0.19.0 to 0.20.0.

Release notes

Sourced from asyncpg's releases.

asyncpg v0.20.0

Improvements

• Support Python 3.8
  (by @​1st1 in #504)

• Support PgBouncer by sending only a single SYNC message per query
  (by @​fvannee in b043fbd3)

Bug Fixes

• Handle IP values with prefix in "inet" type as ipaddress.IPvXInterface
  (by @​elprans in 5a4daf71 for #497)

• Close transport if connection times out
  (by @​hexrain in 926f4833 for #468)

• Use faster UUID codecs; make UUID decoding/encoding/operations 2-7x faster
  (by @​1st1 in edde3ff4)

• Use loop.start_tls() to upgrade connections to SSL
  (by @​1st1 in bdba7ce7)

Build

• Bump Cython to 0.29.14
  (by @​1st1 in 7cb31bc6)

Commits

• ae7c110 Bump the version to 0.20.0
• b3fe95d Fix build wheels testing: don't install [test]
• dcef298 Silence 3.8 warnings about implicit int conversion
• bdba7ce Use loop.start_tls() to upgrade connections to SSL
• d655a39 Skip test_adversity on Windows & Python 3.8
• b8483d2 Handle Stream.wait_closed() windows bug gracefully
• 70b0403 CI: Don't run uvloop tests on built wheels
• fd88731 CI: Enable Python 3.8
• e4f33f5 Adopt new Python 3.8 tuple.hash algo for Record.hash
• 7978b7b Pin flake8 & pycodestyle; fix style issues
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #24.