$\newcommand{\ep}{\epsilon}\newcommand{\de}{\delta}$The [Kullback–Leibler (KL) divergence][2] may be defined by the formula 
\begin{equation*}
	D(P\parallel Q):=KL(P\parallel Q):=\int p\ln\frac pq=\int q\, g(p/q),
\end{equation*}
where $P$ and $Q$ are probability measures on a measurable space; $p$ and $q$ are, respectively, densities of $P$ and $Q$ with respect to a measure $\mu$ such that $P$ and $Q$ are absolutely continuous with respect to $\mu$; $\int f:=\int f\,d\mu$; and 
$$g(u):=u\ln u$$
for $u\in(0,\infty)$, with $g(0):=0$ and $g(\infty):=\infty$. Here we are using the standard conventions $a/0:=\infty$ for $a>0$ and $0\times\text{anything}=\text{anything}\times0:=0$. 
For $\mu$, one always take e.g. $P+Q$. 
It is easy to see and very well known that we always have $D(P\parallel Q)\in[0,\infty]$. 

Here, it is given that 
\begin{equation*}
	c:=D(P\parallel Q)<\infty. 
\end{equation*}
Without loss of generality $c>0$ (otherwise, there is nothing to prove). 
Take any $\ep\in(0,3c/2]$, so that 
\begin{equation*}
	\de:=\ep/(3c)\in(0,1/2]. 
\end{equation*}
Take now any natural $n\ge2$ and for $j\in[n]:=\{1,\dots,n\}$ let 
\begin{equation*}
	R_j:=P_{t_j},
\end{equation*}
where 
\begin{equation*}
	P_t:=(1-t)P+tQ
\end{equation*}
and 
\begin{equation*}
	t_j:=\de+\frac{j-1}{n-1}\,(1-2\de), 
\end{equation*}
so that $t_1=\de\le1-\de=t_n$, $P_0=P$, $R_1=P_\de$, $R_n=P_{1-\de}$, and $P_1=Q$. 

By the obvious convexity of $D(P\parallel Q)$ in $P$ and in $Q$, for all $t\in[0,1]$
\begin{equation*}
	D(P\parallel P_t)\le(1-t)D(P\parallel P_0)+tD(P\parallel P_1)=tc
\end{equation*}
and 
\begin{equation*}
	D(P_t\parallel Q)\le(1-t)D(P_0\parallel Q)+tD(P_1\parallel Q)=(1-t)c. 
\end{equation*}
So, 
\begin{equation*}
D(P\parallel R_1)\le\ep/3,\quad D(R_n\parallel Q)\le\ep/3. 
\end{equation*}

To bound $D(R_j\parallel R_{j+1})$ for $j\in[n-1]$, we will use 

>**Lemma 1:** For any $s$ and $t$ in $[\de,1-\de]$, 
\begin{equation}
	D(P_s\parallel P_t)\le\frac{2(s-t)^2}\de.  
\end{equation}

This lemma will be proved at the end of the answer. At this point, just note that, by Lemma 1, 
\begin{equation}
	D(R_j\parallel R_{j+1})\le\frac{2(1-2\de)^2}{(n-1)^2\de}
\end{equation}
for $j\in[n-1]$, whence 
\begin{equation}
	D(P\parallel R_1)+D(R_1\parallel R_2)+\dots+D(R_{n-1}\parallel R_n)+D(R_n\parallel Q) \\ 
	\le\ep/3+\frac{2(1-2\de)^2}{(n-1)\de}+\ep/3<\ep,
\end{equation}
as desired, if $n$ is taken to be large enough. 

It remains to provide 

*Proof of Lemma 1:* Note that $g(u)\le u-1+(u-1)^2$ for all real $u>0$. So, letting $p_t:=(1-t)p+tq$ for $t\in[0,1]$, so that $p_t$ is the density of $P_t$, we have 
\begin{align*}
	D(P_s\parallel P_t)&=\int p_t g(p_s/p_t) \\ 
	&\le\int p_t \Big[\frac{p_s}{p_t}-1+\Big(\frac{p_s}{p_t}-1\Big)^2\Big] \\ 
	&\le\int p_t \Big(\frac{p_s}{p_t}-1\Big)^2 \\ 
	&=\int \frac{(p_s-p_t)^2}{p_t} \\ 
	&=(s-t)^2\int \frac{(p-q)^2}{(1-t)p+tq} \\ 
	&\le\frac{(s-t)^2}\de\,\int \frac{(p-q)^2}{p+q} \\ 
	&\le\frac{(s-t)^2}\de\,\int(p+q)=\frac{2(s-t)^2}\de. \quad\Box 
\end{align*}

[2]: https://en.wikipedia.org/wiki/Kullback%E2%80%93Leibler_divergence