drop python 3.7

```
pygit2<=0.28.2; python_version < '3.8'
pygit2>=1.2.0; python_version >= '3.8'
```

Author: chenrui333

.appveyor.yml

@@ -1,10 +1,6 @@
 # Based on pygit2's appveyor config
 environment:
   matrix:
-  - GENERATOR: 'Visual Studio 14'
-    PYTHON: 'C:\Python37\python.exe'
-  - GENERATOR: 'Visual Studio 14 Win64'
-    PYTHON: 'C:\Python37-x64\python.exe'
   - GENERATOR: 'Visual Studio 14'
     PYTHON: 'C:\Python38\python.exe'
   - GENERATOR: 'Visual Studio 14 Win64'

.travis.yml

@@ -5,7 +5,6 @@ addons:
     packages:
     - snapcraft
 python:
-- '3.7'
 - '3.8'
 env: LIBGIT2=~/libgit2/_install/ LD_LIBRARY_PATH=~/libgit2/_install/lib
 before_install: ./.travis.sh