Drop Python 3.7 Support

[marvinpfoertner]

In a Nutshell

NumPy v1.22 dropped support for Python 3.7 (see numpy/numpy#19665). Let's do the same for ProbNum.

Detailed Description

• require Python 3.8 as the minimal Python version
• update CI
• remove the conditional dependency on cached_property for Python <3.8 and remove the corresponding conditional imports

Related Issues

None

[codecov]

Codecov Report

Merging #632 (2c48601) into main (eaf86d4) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
+ Coverage   89.56%   89.78%   +0.22%     
==========================================
  Files         181      181              
  Lines        6863     6836      -27     
  Branches     1062     1062              
==========================================
- Hits         6147     6138       -9     
+ Misses        485      467      -18     
  Partials      231      231              

Impacted Files	Coverage Δ
...um/linalg/solvers/beliefs/_linear_system_belief.py	90.32% <100.00%> (+2.63%)	⬆️
src/probnum/randprocs/markov/integrator/_ioup.py	100.00% <100.00%> (+2.81%)	⬆️
src/probnum/randprocs/markov/integrator/_iwp.py	100.00% <100.00%> (+2.40%)	⬆️
src/probnum/randprocs/markov/integrator/_matern.py	100.00% <100.00%> (+2.70%)	⬆️
...num/randprocs/markov/integrator/_preconditioner.py	100.00% <100.00%> (+5.55%)	⬆️
src/probnum/randvars/_constant.py	89.36% <100.00%> (+3.36%)	⬆️
src/probnum/randvars/_normal.py	82.43% <100.00%> (+0.65%)	⬆️
src/probnum/randvars/_random_variable.py	63.41% <100.00%> (+0.31%)	⬆️
src/probnum/randvars/_randomvariablelist.py	86.66% <100.00%> (+2.53%)	⬆️