Fix UnboundLocalError in _get_version() when package metadata is missing

[yashbudhia]

Client._get_version() has a fallback for when the package metadata cannot be found, but the fallback never runs. Instead every request raises UnboundLocalError.

Reproduce (any environment where razorpay is importable but not installed, e.g. a source checkout or a vendored copy):

>>> import importlib.metadata, razorpay
>>> importlib.metadata.version = lambda name: (_ for _ in ()).throw(importlib.metadata.PackageNotFoundError(name))
>>> razorpay.Client(auth=("k", "s"))._get_version()
UnboundLocalError: cannot access local variable 'DistributionNotFound' where it is not associated with a value

Cause

DistributionNotFound is imported only inside the pkg_resources branch, but it is named in the except (PackageNotFoundError, DistributionNotFound, NameError) clause that also guards the importlib.metadata path. When importlib.metadata.version() raises PackageNotFoundError, Python evaluates that tuple, hits the unbound local, and the new UnboundLocalError replaces the original exception. It is raised while evaluating the except expression, so the clause itself cannot catch it, and it propagates out of _update_user_agent_header() and therefore out of every API call.

Fix

Resolve the "not found" exception class before doing the lookup, then look up the version in its own try. Behaviour when the package is installed is unchanged; when the metadata is missing the documented fallback version is returned with the existing UserWarning. The fallback value itself is left as it was.

Tests

tests/test_client_version.py covers the installed path, the fallback with its warning, and that a request's User-Agent header can still be built without metadata. Full suite: 160 passed, 1 skipped.