Drop python 3.7

[jenshnielsen]

Now that QCoDeS has dropped support for 3.7 lets drop it here too

[codecov-commenter]

Codecov Report

Merging #166 (c7345e6) into master (281fd41) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   18.84%   18.80%   -0.04%     
==========================================
  Files         123      123              
  Lines       15539    15581      +42     
==========================================
+ Hits         2928     2930       +2     
- Misses      12611    12651      +40     

Impacted Files	Coverage Δ
...s_contrib_drivers/drivers/SignalCore/SignalCore.py	0.00% <0.00%> (ø)
qcodes_contrib_drivers/drivers/Oxford/kelvinox.py	0.00% <0.00%> (ø)
...contrib_drivers/drivers/ERAInstruments/erasynth.py	0.00% <0.00%> (ø)
...trib_drivers/drivers/Cryogenic/CryogenicSMS120C.py	0.00% <0.00%> (ø)
..._drivers/drivers/NationalInstruments/visa_types.py	0.00% <0.00%> (ø)
...drivers/drivers/Keysight/SD_common/SD_AWG_Async.py	0.00% <0.00%> (ø)
qcodes_contrib_drivers/drivers/QDevil/QDAC2.py	96.47% <0.00%> (+<0.01%)	⬆️
...ivers/drivers/Keysight/SD_common/memory_manager.py	80.17% <0.00%> (+0.17%)	⬆️

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