Add support for Python 3.9 build testing (#1018)

* First stab at adding Python 3.9 dev Travis builds
* Dynamically replace Python max version for Python 3.9 beta builds
* Fix allow_failures (3.9-dev, not 3.9) and fix Windows find-and-replace command to allow 3.9
* Fix allow_failures to allow all three Python 3.9 builds to fail
* Use platform-independent Python script to allow all Python versions for 3.9 builds
* Format changes and changing before_install to script for 3.9 builds
* Install toml module to make it possible to run version-changing script before install
* Properly populate before_install step with Mac and Windows Python 3.9, and remove extraneous unused python key
* Add refreshenv to get python3 path included in WIndows
* Ensure MacOS pyenv is up to date and attempt to reference Windows python3 directly
* Use bash-friendly paths in Windows and check to see exactly what the Python 3.9 executable is
* Confirm Python version in Windows and attempt to get pip a different way
* Fiddling with Windows python references
* Remove Python3.6 references and check for Python directory in root directory
* Add Python 3.9 path to Windows build
* Remove pip steps that should be unnecessary
* Add upgrade pip to Windows Python 3.9 build
* Attempt to resolve "access denied" error when upgrading pip on Windows
* Use allow_all_python_version TOML script in nightly build instead of sed

.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)