Switch to Poetry for dependency and build management

[rzvoncek]

Fixes #555.
Fixes #667.
Fixes #665.
Fixes #712.
Fixes #706.

In attempt to stay somewhat recent, this PR switches Medusa's dependencies and (Python) packaging to be managed by Poetry. The debian package build is updated to reflect that too.

• The CI is green.
• Building the deb package on a fresh focal (20.04) works to a point that installing that package on another focal node works too (did a backup with it).
• Installing medusa with pip straight from this branch works too (on a focal instance), but I did not try to do a backup.
• Installing medusa offline works too according to the updated documentation (tested on both ubuntu and RHEL).
• I've also managed to push medusa into the test pypi. I could not install it from there because all the deps are not in the test pypi.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.14%. Comparing base (b0cc958) to head (6ccef08).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #679   +/-   ##
=======================================
  Coverage   80.14%   80.14%           
=======================================
  Files          52       52           
  Lines        4708     4708           
  Branches      958      958           
=======================================
  Hits         3773     3773           
  Misses        906      906           
  Partials       29       29           

Files	Coverage Δ
medusa/service/grpc/restore.py	44.82% <ø> (ø)

[sonarqubecloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

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[adejanovski]

Works great 👍

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 Security Hotspots

See analysis details on SonarCloud