How would I build python myself from source code on Ubuntu?

You may try using pyenv. I haven't tried it yet. But looking at the sources, it seems very mature to accomplish an installation of any CPython-interpreter on any *ix-system.

1. At a shell prompt (in a terminal), run

   sudo apt-get install build-essential 
   

   This will fetch all the common packages you need to build anything (e.g. the compiler etc.).

2. Then run

   sudo apt-get build-dep python2.7
   

   This will fetch all the libraries you need to build python.

3. Then download the source code for python and decompress it into a directory.

4. go there and run

   ./configure --prefix=/path/where/you/want/python/installed
   

5. Then make and then make install to get it built and installed:

   make && make install
   

If you hit snags on the way, ask back here and I'll try to offer some guidance.

The best way to build "hot" very recent python (from github) is as follows:

  sudo apt-get update \
  && sudo apt-get install -y build-essential git libexpat1-dev libssl-dev zlib1g-dev \
  libncurses5-dev libbz2-dev liblzma-dev \
  libsqlite3-dev libffi-dev tcl-dev linux-headers-generic libgdbm-dev \
  libreadline-dev tk tk-dev

  git clone https://github.com/python/cpython.git
  cd cpython && ./configure --prefix=/usr \
  --enable-loadable-sqlite-extensions \
  --enable-shared \
  --with-lto \
  --enable-optimizations \
  --with-system-expat \
  --with-system-ffi \
  --enable-ipv6 --with-threads --with-pydebug --disable-rpath \
  && make \
  && sudo make install

It builds the very recent python from the sources on github.

With this I have built Python 3.8.0a0 (heads/master:077059e0f0, Aug 10 2018, 21:36:32).