move code to /src/ folder pattern

[gjedlicska]

Motivation:

https://hynek.me/articles/testing-packaging/

[codecov]

Codecov Report

Base: 90.85% // Head: 90.75% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (9585d46) compared to base (fd09e97).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
- Coverage   90.85%   90.75%   -0.11%     
==========================================
  Files          54       54              
  Lines        3850     3850              
==========================================
- Hits         3498     3494       -4     
- Misses        352      356       +4     

Impacted Files	Coverage Δ
src/specklepy/api/client.py	85.52% <ø> (ø)
src/specklepy/api/credentials.py	76.05% <ø> (ø)
src/specklepy/api/host_applications.py	94.44% <ø> (ø)
src/specklepy/api/models.py	91.26% <ø> (ø)
src/specklepy/api/operations.py	93.47% <ø> (ø)
src/specklepy/api/resource.py	89.83% <ø> (ø)
src/specklepy/api/resources/__init__.py	100.00% <ø> (ø)
src/specklepy/api/resources/branch.py	100.00% <ø> (ø)
src/specklepy/api/resources/commit.py	91.11% <ø> (ø)
src/specklepy/api/resources/object.py	100.00% <ø> (ø)
... and 30 more

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[AlanRynne]

Seems legit! 🙌🏼