BLD: Bump requirements to python 3.9

I started to use type annotations that are available only in 3.9
Since 3.8 reaches end of life in october and new versions of numpy
are targetting 3.9, I think it's safe to upgrade the minimum
version here.

.github/workflows/pull-docs.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ "3.7" ]
+        python-version: [ "3.9" ]
         poetry-version: [ "1.2.1" ]
     steps:
       # ======

.github/workflows/push-test.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.7"]
+        python-version: ["3.9"]
         poetry-version: ["1.2.1"]
 
     steps:

.github/workflows/release.yml

@@ -14,7 +14,7 @@ jobs:
       name: release
     strategy:
       matrix:
-        python-version: ["3.7"]
+        python-version: ["3.9"]
         poetry-version: ["1.2.1"]
     steps:
       # ======

pyproject.toml

@@ -15,7 +15,7 @@ readme = "README.rst"
 
 
 [tool.poetry.dependencies]
-python = "^3.7"
+python = "^3.9"
 numpy = "^1.18.3"
 scipy = "^1.4.1"
 scikit-learn = "^1"