Unpin sympy and ignore sympy type errors (quantumlib#5226)

Fixes quantumlib#5058
Review: @dabacon

Author: maffoo

cirq-core/requirements.txt

@@ -13,7 +13,6 @@ numpy~=1.16
 pandas
 sortedcontainers~=2.0
 scipy
-# TODO: unpin once #5058 is resolved
-sympy<1.10
+sympy
 typing_extensions
 tqdm

dev_tools/conf/mypy.ini

@@ -17,7 +17,8 @@ follow_imports_for_stubs = true
 
 #Adding "sympy.* or mypy-sympy to the above list (3rd-party libs for which we don't have stubs) doesn't ignore "cannot find module 'sympy' error
 [mypy-sympy.*]
-ignore_missing_imports = True
+follow_imports = skip
+ignore_missing_imports = true
 
 # Treat symbols imported from Google's protobuf library as type Any.
 # This supresses errors due to attributes not known to typeshed,