Update pyautoversion requirement from ~=1.2.0 to >=1.2,<2.1

Updates the requirements on [pyautoversion](https://github.com/ARMmbed/autoversion) to permit the latest version.
- [Release notes](https://github.com/ARMmbed/autoversion/releases)
- [Changelog](https://github.com/ARMmbed/autoversion/blob/master/CHANGELOG.md)
- [Commits](ARMmbed/autoversion@1.2.0...2.0.1)

---
updated-dependencies:
- dependency-name: pyautoversion
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

setup.py

@@ -53,7 +53,7 @@
     install_requires=[
         "gitpython",
         "towncrier==19.2.0",
-        "pyautoversion~=1.2.0",
+        "pyautoversion>=1.2,<2.1",
         # FIXME change when https://github.com/pdoc3/pdoc/issues/299 is fixed
         "pdoc3==0.9.1",
         "toml",