Use platform-independent Python script to allow all Python versions for 3.9 builds

.build/allow_all_python_version.py

@@ -0,0 +1,8 @@
+import toml
+
+pyproject = toml.load("pyproject.toml")
+
+pyproject['tool']['poetry']['dependencies']['python'] = "*"
+
+with open("pyproject.toml", "w") as toml_file:
+	toml.dump(pyproject, toml_file)

.travis.yml

@@ -111,19 +111,19 @@ jobs:
     # Python 3.9 Dev Tests
     - name: Python 3.9 on Linux
       before_install:
-        - sed -i 's/^python = ">=3\.7\.0.*"$/python = "*"/' pyproject.toml
+        - python .build/allow_all_python_version.py
       python: 3.9-dev
     - <<: *test_mac
       name: Python 3.9 on MacOS
       before_install:
-        - sed -i 's/^python = ">=3\.7\.0.*"$/python = "*"/' pyproject.toml
+        - python .build/allow_all_python_version.py
       python: 3.9-dev
       env:
         JRNL_PYTHON_VERSION: 3.9.0b5
     - <<: *test_windows
       name: Python 3.9 on Windows
       before_install:
-        - powershell -Command "(gc pyproject.toml) -replace 'python = "">=3.7.0, <3.9.0""', 'python = "">=3.7.0, <3.10.0""' -join \"`n\" | Out-File -encoding UTF8 pyproject.toml"
+        - python .build/allow_all_python_version.py
       python: 3.9-dev
       env:
         <<: *env_windows