add: partial copy method

[TOsmanov]

Workflows:

• Tests and lint are separated

Base Foliant backend:

• Added the partial_copy method, which copies files according to a given list
• Added tests

CLI:

• Added the prepare_list_of_files method, which prepares a list of files from a user-specified string

Utils:

• Updated for compatibility with Python 3.9

Other

• Python 3.9 is now the minimum required version
• Added scripts for running tests and for running tests in Docker

[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.81%. Comparing base (9a39e1f) to head (15b616d).

Files with missing lines	Patch %	Lines
foliant/partial_copy.py	76.57%	41 Missing ⚠️
foliant/cli/make.py	90.47%	2 Missing ⚠️
foliant/backends/base.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   76.88%   77.81%   +0.92%     
==========================================
  Files          14       15       +1     
  Lines         398      613     +215     
==========================================
+ Hits          306      477     +171     
- Misses         92      136      +44     

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