Remove pypy3 from .travis.yml

[texasaggie97-zz]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Removes pypy3 from .travis.yml since we no longer explicitly test it.

List issues fixed by this Pull Request below, if any.

What testing has been done?

• Unit

[codecov]

Codecov Report

Merging #1285 into master will decrease coverage by 4.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
- Coverage   91.53%   87.49%   -4.05%     
==========================================
  Files          20       26       +6     
  Lines        3674     5748    +2074     
==========================================
+ Hits         3363     5029    +1666     
- Misses        311      719     +408     

Flag	Coverage Δ
#codegenunittests	87.74% <ø> (ø)	⬆️
#nidcpowersystemtests	98.58% <ø> (?)
#nidigitalsystemtests	51.72% <ø> (?)
#nifakeunittests	96.09% <ø> (ø)	⬆️
#nifgensystemtests	96.91% <ø> (?)
#nimodinstsystemtests	87.23% <ø> (?)
#nimodinstunittests	96.01% <ø> (ø)	⬆️
#nisesystemtests	100.00% <ø> (?)
#nitclksystemtests	100.00% <ø> (?)
#nitclkunittests	95.94% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
generated/nise/nise/_library.py	100.00% <0.00%> (ø)
generated/nidigital/nidigital/_library.py	51.72% <0.00%> (ø)
generated/nimodinst/nimodinst/_library.py	87.23% <0.00%> (ø)
generated/nitclk/nitclk/_library.py	100.00% <0.00%> (ø)
generated/nifgen/nifgen/_library.py	96.91% <0.00%> (ø)
generated/nidcpower/nidcpower/_library.py	98.58% <0.00%> (ø)

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