fix: uproot package name migration to uproot3

[alexander-held]

In preparation for the migration uproot4 -> uproot, the "old" uproot migrates to uproot3. Since uproot3 is available now (https://pypi.org/project/uproot3/), this updates uproot -> uproot3. The migration uproot4 -> uproot will follow later.

Also adding upper limits for the pyhf and iminuit versions. Compatibility with pyhf 0.6 will require updates to cabinetry (#150/#156), and the interface changes in iminuit 2.0 will also require an update to cabinetry.

[codecov]

Codecov Report

Merging #157 (a28411f) into master (becf994) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #157   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          13       13           
  Lines        1471     1471           
  Branches      211      211           
=======================================
  Hits         1441     1441           
  Misses         28       28           
  Partials        2        2           

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