Remove environment marker for backports.zoneinfo when using Bazel.

The `rules_python` `requirement` macro only does name-mangling. To handle
an environment marker for a py_library dependency we will need something more
sophisticated (e.g. a macro that accounts for the interpreter version).

Since Bazel requirements already force the user to have the tools necessary to
build a backports.zoneinfo whl, for now we'll keep this pip-dependency for
Bazel, but have a PEP 508 environment marker for our pypi distribution.

PiperOrigin-RevId: 380644314

bazel/requirements.txt

@@ -1,5 +1,5 @@
 absl-py~=0.10.0
-backports.zoneinfo~=0.2.1;python_version<"3.9"
+backports.zoneinfo~=0.2.1
 protobuf~=3.13.0
 python-dateutil~=2.8.1
 requests~=2.24.0