Implement SQLite backups with the .backup command

[whileman133]

Description

The SqliteCPConnector is unsafe to use when there are active connections to the SQLite3 database (see How To Corrupt An SQLite Database File at sqlite.org). This PR adds SqliteBackupConnector that uses the .backup command in SQLite to safely backup a SQLite3 database with active connections.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.29%. Comparing base (515a099) to head (76d164a).

Files with missing lines	Patch %	Lines
dbbackup/db/sqlite.py	84.21%	2 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
- Coverage   91.43%   91.29%   -0.15%     
==========================================
  Files          19       19              
  Lines         934      953      +19     
  Branches      138      139       +1     
==========================================
+ Hits          854      870      +16     
- Misses         44       46       +2     
- Partials       36       37       +1     

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