Pin pyproj to 3.4.1, 3.6.1 fails to build in docker

Our base image doesn't install a recent enough
version of proj-bin, so we'd need to install from
source. I can't see any reason to do this as yet.

requirements.txt

@@ -5,7 +5,7 @@ pyparsing>=2.1.10
 requests>=1.1.0
 cython==0.29.36; python_version < '3.9'
 pyproj==2.6.1; python_version < '3.9'
-pyproj==3.6.1; python_version >= '3.9'
+pyproj==3.4.1; python_version >= '3.9'
 
 Shapely==2.0.1
 OWSLib==0.28.1