build: drop explicit awkward dependency

[alexander-held]

Following #408 there was only one place left where awkward was used in the cabinetry code base, in cabinetry.tabulate. The use of awkward there was not crucial, so this is replaced by pure numpy here. This means that the explicit awkward dependency can be dropped from cabinetry. It still enters in cabinetry[contrib] via uproot.

* drop explicit dependency on awkward
* remove awkward HEAD tests from nightly CI tests

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7b9b6af) 100.00% compared to head (d5056f0) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #419   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2072      2072           
  Branches       337       338    +1     
=========================================
  Hits          2072      2072           

Impacted Files	Coverage Δ
src/cabinetry/tabulate.py	100.00% <100.00%> (ø)

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