Add PHP8.0 support

[johnzuk]

close #97

[codecov-io]

Codecov Report

Merging #98 (dff8681) into master (d4c023a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #98   +/-   ##
=========================================
  Coverage     92.54%   92.54%           
  Complexity      188      188           
=========================================
  Files            31       31           
  Lines           456      456           
=========================================
  Hits            422      422           
  Misses           34       34           

Impacted Files	Coverage Δ	Complexity Δ
src/GusApi/Client/GusApiClient.php	92.98% <100.00%> (ø)	19.00 <2.00> (ø)
src/GusApi/GusApi.php	100.00% <100.00%> (ø)	33.00 <2.00> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d4c023a...dff8681. Read the comment docs.