fix for incorrect stats computation in diff exp t-test (#2318)

* 2211 fixes

* lint

* lint

* add missing test and bug found by test

* change terminology for count distribution

* update scanpy requirement

* update scanpy requirement

backend/server/requirements.txt

@@ -1,4 +1,4 @@
-anndata>=0.7.0
+anndata>=0.7.6 # we need to_memory(), added in 0.7.6
 boto3>=1.12.18
 click>=7.1.2
 Flask>=1.0.2,<2.0.0 # Flask 2.0 is not compatible with the latest version of Flask-RESTful (0.3.8)
@@ -11,9 +11,9 @@ flatbuffers>=1.11.0,<2.0.0 # cellxgene is not compatible with 2.0.0. Requires mi
 flatten-dict>=0.2.0
 fsspec>=0.4.4,<0.8.0
 gunicorn>=20.0.4
-h5py<3.0.0 # h5py>=3.0.0 had a breaking change; there is a fix in anndata>=0.7.5
+h5py>=3.0.0
 jinja2>=2.11.3  # Flask sub-dependency. Added due to CVE-2020-28493
-numba>=0.51.2,<0.53.0
+numba>=0.51.2
 numpy>=1.17.5
 packaging>=20.0
 pandas>=1.0,!=1.1  # pandas 1.1 breaks tests, https://github.com/pandas-dev/pandas/issues/35446