bumped version and moved version to 1 place

[MauriceHendrix]

Description

• Bumped version in preparation for new release.
• Moved version number to version.txt which is read from setup.py and docs/config.py
• Also added functionality to cellmlmanip to report its own version via version() and version + tests
• Also bumped pytest version

Motivation and Context

• If the version number is just in 1 place, there is less of a risk of forgetting to bump especially the docs version number.
• Pytest version bump was needed to make non-pinned tests pass.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Documentation checklist

• [X] I have updated all documentation in the code where necessary.
• [X] I have checked spelling in all (new) comments and documentation.
• [ ] I have added a note to RELEASE.md if relevant (new feature, breaking change, or notable bug fix).

Testing

• [x] Testing is done automatically and codecov shows test coverage
• [ ] This cannot be tested automatically

[codecov[bot]]

Codecov Report

Merging #306 into develop will increase coverage by 0.01%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #306      +/-   ##
===========================================
+ Coverage    98.60%   98.61%   +0.01%     
===========================================
  Files            6        8       +2     
  Lines         1361     1376      +15     
  Branches       299      302       +3     
===========================================
+ Hits          1342     1357      +15     
  Misses           7        7              
  Partials        12       12              

Impacted Files	Coverage Δ
cellmlmanip/__init__.py	100.00% <100.00%> (ø)
cellmlmanip/_config.py	100.00% <100.00%> (ø)

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