Need Python 3.7

Author: jeromekelleher

docs/installation.rst

@@ -4,7 +4,7 @@
 Installation
 ############
 
-Python 3.6 or newer is required for ``tsinfer``. Any Unix-like platform should
+Python 3.7 or newer is required for ``tsinfer``. Any Unix-like platform should
 work (``tsinfer`` is tested on Linux, OS X, and Windows).
 
 ***************
@@ -67,4 +67,4 @@ Potential issues
    (see https://github.com/tskit-dev/tsinfer/issues/376). The current workaround is
    either to compile from source by installing alternative python and C compilers via
    conda (``conda install -c conda-forge c-compiler``) or to install the binary
-   packages via conda as recommended at the top of this page.
+   packages via conda as recommended at the top of this page.