build: Update lower bound on tensorflow to v2.6.5 (#1874)

* Update lower bound on tensorflow to v2.6.5 as it is the "oldest" SemVer release
that is not broken on install by protobuf v4.21.0 ABI incompatibility.
   - c.f. tensorflow/tensorflow#56077
   - Amend PR #1869
* Update tests/constraints.txt to use tensorflow==2.6.5.

setup.py

@@ -3,7 +3,7 @@
 extras_require = {
     'shellcomplete': ['click_completion'],
     'tensorflow': [
-        'tensorflow>=2.3.1',  # c.f. https://github.com/tensorflow/tensorflow/pull/40789
+        'tensorflow>=2.6.5',  # c.f. PR #1874
         'tensorflow-probability>=0.11.0',  # c.f. PR #1657
     ],
     'torch': ['torch>=1.10.0'],  # c.f. PR #1657

tests/constraints.txt

@@ -11,8 +11,7 @@ uproot==4.1.1
 # minuit
 iminuit==2.4.0
 # tensorflow
-tensorflow==2.3.1  # tensorflow-probability v0.11.0 requires tensorflow>=2.3
-protobuf<=3.20.1  # protobuf v4.21.0 breaks ABI compatability
+tensorflow==2.6.5  # c.f. PR #1874
 tensorflow-probability==0.11.0  # c.f. PR #1657
 # torch
 torch==1.10.0