build(python): install google-api-core for python generator (#916)

requirements.txt

@@ -1,4 +1,3 @@
-# Pin to less than 49.6.0 as a workaround for https://github.com/pypa/setuptools/issues/2353
 setuptools==50.3.2
 
 nox
@@ -9,6 +8,12 @@ deprecation
 protobuf==3.14.0
 watchdog
 
+# https://github.com/googleapis/gapic-generator/issues/3334
+# temporarily install google-api-core for gapic-generator-python
+# Bazel should install this, but there is currently a bug
+# that causes the package to be skipped
+google-api-core<2.0.0
+
 # some java processing requires xml handling
 lxml