opentelemetry-test-utils: skip installing grpc exporter only on Pypy (#5178)

* opentelemetry-test-utils: move requirements environment markers to a constraint file

For some reason dependabot fails parsing them.

* Second try

* Apply suggestions from code review

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>

* Update tox.ini

* Update dependency paths in test-requirements.txt

Change paths for dependencies in test-requirements.txt

* Skip only on pypy since installation of requirements on windows seems to work

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>