Pin pip to 19.3.1 on Windows

pip 20 made the following change:

> Remove interpreter-specific major version tag e.g. cp3-none-any from consideration.
> https://github.com/pypa/pip/pull/7355

The coincurve pip package only has a major version tag for its windows
wheel. pip20 will not allow us to install that wheel, and compiling
coincurve from source causes errors. So we cannot use pip 20 or above
until coincurve fixes its compatibility tags, we can compile coincurve
from source, we stop using coincurve, or we figure out another way to
install it.

Note that on osx and linux, coincurve has the correct tags.

.gitlab-ci.yml

@@ -118,11 +118,12 @@ build:windows:
   before_script:
     - ./docker/install_choco.ps1
     - choco install -y --x86 python3 7zip checksum
-#    - python --version | findstr /B "Python 3\.7\." # dont upgrade python on every run. just make sure we're on the right Python
-#    - pip --version | findstr /E '\(python 3\.7\)'
+    - python --version # | findstr /B "Python 3\.7\." # dont upgrade python on every run. just make sure we're on the right Python
+    - pip --version # | findstr /E '\(python 3\.7\)'
     - pip install virtualenv pywin32
     - virtualenv venv
     - venv/Scripts/activate.ps1
+    - pip install pip==19.3.1; $true # $true ignores errors. need this to get the correct coincurve wheel. see commit notes for details.
   after_script:
     - rmdir -Recurse venv
   script: