Loosen up the dependency on `backoff`.

[kaos]

When trying to install opentelemetry-exporter-otlp-proto-gprc we hit a dependency issue involving backoff.

ERROR: Cannot install -r requirements.txt (line 42), backoff==1.11.1 and opentelemetry-exporter-otlp because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested backoff==1.11.1
    <redacted> 6.18.0 depends on backoff<2.0.0 and >=1.11.1
    opentelemetry-exporter-otlp-proto-grpc 1.8.0 depends on backoff~=1.10.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

opentelemetry-python/exporter/opentelemetry-exporter-otlp-proto-grpc/setup.cfg

Line 48 in 1a52e9d

backoff ~= 1.10.0

Is there an issue with backoff 1.11.1 that makes it unusable for this project?
If not, I'd suggest using a less restrictive requirement, that allows minor and patch upgrades.

[srikanthccv]

Our usage of this lib is only limited to definition expo. I don't see any breaking changes to that since the initial release of package. Would appreciate if you can test and send a PR Loosening up the requirement.

[kaos]

Would appreciate if you can test and send a PR Loosening up the requirement.

OK! Thanks, I'll see if I can do that when I get some spare time, unless someone else beats me to it (not going to be too soon). I've worked around this by pinning our deps to backoff 1.10.0 as well for now.

[teddylear]

@aabmass @srikanthccv Made small PR to address this. Looks like I need approval fro CI jobs as this is my first PR to the project. Please let me know if I should change anything, thanks!