chore: release 2.2.1b1

[release-please]

🤖 I have created a release *beep* *boop*

2.2.1b1 (2021-06-17)

Features

• Add support for open telemetry (#633) (2ba879a)
• add decimal/numeric support (#620) (d09ad61)
• added unit test with coverage of 68% (#611) (92ad508)
• update docs and nox file to compile it (#610) (876f2fc)
• update workflow files to uniformly distribute the test modules by time taken by each test module (#615) (c386123)
• move migrations test modules to run against different emulator (#613) (5b3b2e4)

Bug Fixes

• correct import path (#649) (bc99bb5)
• remove error msg check from test_decimal_precision_limit for non emulator test (#647) (fe4a062)
• remove error msg check from test_decimal_precision_limit (#646) (9be15c0)
• remove delete of instance if it already exists, as it might be in use by another test (#641) (0544208)
• correct test case for sql_flush for multiple delete table commands (#629) (3de1a81)
• iexact lookup with Transform expression crash issue when RHS is direct value and a transform function is involved (#628) (2772b57)
• Update links in comments to use googleapis repo (#622) (3fa1aeb)
• Remove un necessary file from code base (#608) (58b9969)
• lint_setup_py was failing in Kokoro is now fixed (#607) (d125148)
• Replace, fast-forward Django CI branch (#598) (5d65e3f)

Code Refactoring

• update coverage score in nox file and cleanup imports in test_operations (#638) (b1f49f7)

Documentation

• add docs build kokoro config for django spanner (#644) (ecf241a)

Miscellaneous Chores

• release 2.2.1b1 (505d3ac)
• add SECURITY.md (#616) (75f1a65)
• add a Code of Conduct (#604) (c996400)
• Remove README note about Variance/StdDev (#601) (4ec363f)
• Add contributing section to README (#600) (2311854)
• README updates (#599) (2c8fb24)

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This PR was generated with Release Please. See documentation.

[vi3k6i5]

LGTM

[yoshi-automation]

Release is at https://github.com/googleapis/python-spanner-django/releases/tag/v2.2.1b1